Find sum of all left leaves in a given Binary Tree

Difficulty Level : Easy
Last Updated : 01 Jul, 2021

Given a Binary Tree, find the sum of all left leaves in it. For example, sum of all left leaves in below Binary Tree is 5+1=6.

The idea is to traverse the tree, starting from root. For every node, check if its left subtree is a leaf. If it is, then add it to the result.
Following is the implementation of the above idea.

C++

// A C++ program to find sum of all left leaves
#include <bits/stdc++.h>
using namespace std;
 
/* A binary tree Node has key, pointer to left and right
   children */
struct Node
{
    int key;
    struct Node* left, *right;
};
 
/* Helper function that allocates a new node with the
   given data and NULL left and right pointer. */
Node *newNode(char k)
{
    Node *node = new Node;
    node->key = k;
    node->right = node->left = NULL;
    return node;
}
 
// A utility function to check if a given node is leaf or not
bool isLeaf(Node *node)
{
   if (node == NULL)
       return false;
   if (node->left == NULL && node->right == NULL)
       return true;
   return false;
}
 
// This function returns sum of all left leaves in a given
// binary tree
int leftLeavesSum(Node *root)
{
    // Initialize result
    int res = 0;
 
    // Update result if root is not NULL
    if (root != NULL)
    {
       // If left of root is NULL, then add key of
       // left child
       if (isLeaf(root->left))
            res += root->left->key;
       else // Else recur for left child of root
            res += leftLeavesSum(root->left);
 
       // Recur for right child of root and update res
       res += leftLeavesSum(root->right);
    }
 
    // return result
    return res;
}
 
/* Driver program to test above functions*/
int main()
{
    // Let us a construct the Binary Tree
    struct Node *root         = newNode(20);
    root->left                = newNode(9);
    root->right               = newNode(49);
    root->right->left         = newNode(23);
    root->right->right        = newNode(52);
    root->right->right->left  = newNode(50);
    root->left->left          = newNode(5);
    root->left->right         = newNode(12);
    root->left->right->right  = newNode(12);
    cout << "Sum of left leaves is "
         << leftLeavesSum(root);
    return 0;
}

Java

// Java program to find sum of all left leaves
class Node
{
    int data;
    Node left, right;
  
    Node(int item)
    {
        data = item;
        left = right = null;
    }
}
  
class BinaryTree
{
    Node root;
  
    // A utility function to check if a given node is leaf or not
    boolean isLeaf(Node node)
    {
        if (node == null)
            return false;
        if (node.left == null && node.right == null)
            return true;
        return false;
    }
  
     // This function returns sum of all left leaves in a given
     // binary tree
    int leftLeavesSum(Node node)
    {
        // Initialize result
        int res = 0;
  
        // Update result if root is not NULL
        if (node != null)
        {
            // If left of root is NULL, then add key of
            // left child
            if (isLeaf(node.left))
                res += node.left.data;
            else // Else recur for left child of root
                res += leftLeavesSum(node.left);
  
            // Recur for right child of root and update res
            res += leftLeavesSum(node.right);
        }
  
        // return result
        return res;
    }
  
    // Driver program
    public static void main(String args[])
    {
        BinaryTree tree = new BinaryTree();
        tree.root = new Node(20);
        tree.root.left = new Node(9);
        tree.root.right = new Node(49);
        tree.root.left.right = new Node(12);
        tree.root.left.left = new Node(5);
        tree.root.right.left = new Node(23);
        tree.root.right.right = new Node(52);
        tree.root.left.right.right = new Node(12);
        tree.root.right.right.left = new Node(50);
  
        System.out.println("The sum of leaves is " +
                                       tree.leftLeavesSum(tree.root));
    }
}
  
// This code is contributed by Mayank Jaiswal

Python

# Python program to find sum of all left leaves
 
# A Binary tree node
class Node:
    # Constructor to create a new Node
    def __init__(self, key):
        self.key = key
        self.left = None
        self.right = None
 
# A utility function to check if a given node is leaf or not
def isLeaf(node):
    if node is None:
        return False
    if node.left is None and node.right is None:
        return True
    return False
 
# This function return sum of all left leaves in a
# given binary tree
def leftLeavesSum(root):
 
    # Initialize result
    res = 0
     
    # Update result if root is not None
    if root is not None:
 
        # If left of root is None, then add key of
        # left child
        if isLeaf(root.left):
            res += root.left.key
        else:
            # Else recur for left child of root
            res += leftLeavesSum(root.left)
 
        # Recur for right child of root and update res
        res += leftLeavesSum(root.right)
    return res
 
# Driver program to test above function
 
# Let us constrcut the Binary Tree shown in the above function
root = Node(20)
root.left = Node(9)
root.right = Node(49)
root.right.left = Node(23)       
root.right.right = Node(52)
root.right.right.left = Node(50)
root.left.left = Node(5)
root.left.right = Node(12)
root.left.right.right = Node(12)
print "Sum of left leaves is", leftLeavesSum(root)
 
# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

C#

using System;
 
// C# program to find sum of all left leaves
public class Node
{
    public int data;
    public Node left, right;
 
    public Node(int item)
    {
        data = item;
        left = right = null;
    }
}
 
public class BinaryTree
{
    public Node root;
 
    // A utility function to check if a given node is leaf or not
    public virtual bool isLeaf(Node node)
    {
        if (node == null)
        {
            return false;
        }
        if (node.left == null && node.right == null)
        {
            return true;
        }
        return false;
    }
 
     // This function returns sum of all left leaves in a given
     // binary tree
    public virtual int leftLeavesSum(Node node)
    {
        // Initialize result
        int res = 0;
 
        // Update result if root is not NULL
        if (node != null)
        {
            // If left of root is NULL, then add key of
            // left child
            if (isLeaf(node.left))
            {
                res += node.left.data;
            }
            else // Else recur for left child of root
            {
                res += leftLeavesSum(node.left);
            }
 
            // Recur for right child of root and update res
            res += leftLeavesSum(node.right);
        }
 
        // return result
        return res;
    }
 
    // Driver program
    public static void Main(string[] args)
    {
        BinaryTree tree = new BinaryTree();
        tree.root = new Node(20);
        tree.root.left = new Node(9);
        tree.root.right = new Node(49);
        tree.root.left.right = new Node(12);
        tree.root.left.left = new Node(5);
        tree.root.right.left = new Node(23);
        tree.root.right.right = new Node(52);
        tree.root.left.right.right = new Node(12);
        tree.root.right.right.left = new Node(50);
 
        Console.WriteLine("The sum of leaves is " + tree.leftLeavesSum(tree.root));
    }
}
 
  //  This code is contributed by Shrikant13

Javascript

<script>
// Javascript program to find sum of all left leaves
     
    class Node
    {
        constructor(k)
        {
            this.data = k;
            this.left = null;
            this.right = null;
        }
         
    }
     
    // A utility function to check if a given node is leaf or not
    function isLeaf(node)
    {
        if (node == null)
            return false;
        if (node.left == null && node.right == null)
            return true;
        return false;
    }
     
    // This function returns sum of all left leaves in a given
     // binary tree
    function leftLeavesSum(node)
    {
     
        // Initialize result
        let res = 0;
   
        // Update result if root is not NULL
        if (node != null)
        {
         
            // If left of root is NULL, then add key of
            // left child
            if (isLeaf(node.left))
                res += node.left.data;
            else // Else recur for left child of root
                res += leftLeavesSum(node.left);
   
            // Recur for right child of root and update res
            res += leftLeavesSum(node.right);
        }
   
        // return result
        return res;
    }
     
    // Driver program
    root = new Node(20);
    root.left = new Node(9);
    root.right = new Node(49);
    root.left.right = new Node(12);
    root.left.left = new Node(5);
    root.right.left = new Node(23);
    root.right.right = new Node(52);
    root.left.right.right = new Node(12);
    root.right.right.left = new Node(50);
     
    document.write("The sum of leaves is " +leftLeavesSum(root));
         
    // This code is contributed by unknown2108
</script>

Output

Sum of left leaves is 78

Time Complexity: O(N), where n is number of nodes in Binary Tree.

Following is Another Method to solve the above problem. This solution passes in a sum variable as an accumulator. When a left leaf is encountered, the leaf’s data is added to sum. Time complexity of this method is also O(n). Thanks to Xin Tong (geeksforgeeks userid trent.tong) for suggesting this method.

C++

// A C++ program to find sum of all left leaves
#include <bits/stdc++.h>
using namespace std;
 
/* A binary tree Node has key, pointer to left and right
   children */
struct Node
{
    int key;
    struct Node* left, *right;
};
 
/* Helper function that allocates a new node with the
   given data and NULL left and right pointer. */
Node *newNode(char k)
{
    Node *node = new Node;
    node->key = k;
    node->right = node->left = NULL;
    return node;
}
 
/* Pass in a sum variable as an accumulator */
void leftLeavesSumRec(Node *root, bool isleft, int *sum)
{
    if (!root) return;
 
    // Check whether this node is a leaf node and is left.
    if (!root->left && !root->right && isleft)
        *sum += root->key;
 
    // Pass 1 for left and 0 for right
    leftLeavesSumRec(root->left,  1, sum);
    leftLeavesSumRec(root->right, 0, sum);
}
 
// A wrapper over above recursive function
int leftLeavesSum(Node *root)
{
    int sum = 0; //Initialize result
 
    // use the above recursive function to evaluate sum
    leftLeavesSumRec(root, 0, &sum);
 
    return sum;
}
 
/* Driver program to test above functions*/
int main()
{
    // Let us construct the Binary Tree shown in the
    // above figure
    int sum = 0;
    struct Node *root         = newNode(20);
    root->left                = newNode(9);
    root->right               = newNode(49);
    root->right->left         = newNode(23);
    root->right->right        = newNode(52);
    root->right->right->left  = newNode(50);
    root->left->left          = newNode(5);
    root->left->right         = newNode(12);
    root->left->right->right  = newNode(12);
 
    cout << "Sum of left leaves is "
      << leftLeavesSum(root) << endl;
    return 0;
}

Java

// Java program to find sum of all left leaves
class Node
{
    int data;
    Node left, right;
  
    Node(int item) {
        data = item;
        left = right = null;
    }
}
  
// Passing sum as accumulator and implementing pass by reference
// of sum variable
class Sum
{
    int sum = 0;
}
  
class BinaryTree
{
    Node root;
  
    /* Pass in a sum variable as an accumulator */
    void leftLeavesSumRec(Node node, boolean isleft, Sum summ)
    {
        if (node == null)
            return;
  
        // Check whether this node is a leaf node and is left.
        if (node.left == null && node.right == null && isleft)
            summ.sum = summ.sum + node.data;
  
        // Pass true for left and false for right
        leftLeavesSumRec(node.left, true, summ);
        leftLeavesSumRec(node.right, false, summ);
    }
  
    // A wrapper over above recursive function
    int leftLeavesSum(Node node)
    {
        Sum suum = new Sum();
         
        // use the above recursive function to evaluate sum
        leftLeavesSumRec(node, false, suum);
  
        return suum.sum;
    }
  
    // Driver program
    public static void main(String args[])
    {
        BinaryTree tree = new BinaryTree();
        tree.root = new Node(20);
        tree.root.left = new Node(9);
        tree.root.right = new Node(49);
        tree.root.left.right = new Node(12);
        tree.root.left.left = new Node(5);
        tree.root.right.left = new Node(23);
        tree.root.right.right = new Node(52);
        tree.root.left.right.right = new Node(12);
        tree.root.right.right.left = new Node(50);
  
        System.out.println("The sum of leaves is " +
                                    tree.leftLeavesSum(tree.root));
    }
}
  
// This code is contributed by Mayank Jaiswal

Python

# Python program to find sum of all left leaves
 
# A binary tree node
class Node:
 
    # A constructor to create a new Node
    def __init__(self, key):
        self.key = key
        self.left = None
        self.right = None
 
def leftLeavesSumRec(root, isLeft, summ):
    if root is None:
        return
     
    # Check whether this node is a leaf node and is left
    if root.left is None and root.right is None and isLeft == True:
        summ[0] += root.key
 
    # Pass 1 for left and 0 for right
    leftLeavesSumRec(root.left, 1, summ)
    leftLeavesSumRec(root.right, 0, summ)
     
 
# A wrapper over above recursive function
def leftLeavesSum(root):
    summ = [0] # initialize result
     
    # Use the above recursive function to evaluate sum
    leftLeavesSumRec(root, 0, summ)
     
    return summ[0]
 
# Driver program to test above function
 
# Let us construct the Binary Tree shown in the
# above figure
root = Node(20);
root.left= Node(9);
root.right   = Node(49);
root.right.left = Node(23);
root.right.right= Node(52);
root.right.right.left  = Node(50);
root.left.left  = Node(5);
root.left.right = Node(12);
root.left.right.right  = Node(12);
 
print "Sum of left leaves is", leftLeavesSum(root) 
 
# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

C#

using System;
 
// C# program to find sum of all left leaves
public class Node
{
    public int data;
    public Node left, right;
 
    public Node(int item)
    {
        data = item;
        left = right = null;
    }
}
 
// Passing sum as accumulator and implementing pass by reference 
// of sum variable 
public class Sum
{
    public int sum = 0;
}
 
public class BinaryTree
{
    public Node root;
 
    /* Pass in a sum variable as an accumulator */
    public virtual void leftLeavesSumRec(Node node, bool isleft, Sum summ)
    {
        if (node == null)
        {
            return;
        }
 
        // Check whether this node is a leaf node and is left.
        if (node.left == null && node.right == null && isleft)
        {
            summ.sum = summ.sum + node.data;
        }
 
        // Pass true for left and false for right
        leftLeavesSumRec(node.left, true, summ);
        leftLeavesSumRec(node.right, false, summ);
    }
 
    // A wrapper over above recursive function
    public virtual int leftLeavesSum(Node node)
    {
        Sum suum = new Sum();
 
        // use the above recursive function to evaluate sum
        leftLeavesSumRec(node, false, suum);
 
        return suum.sum;
    }
 
    // Driver program
    public static void Main(string[] args)
    {
        BinaryTree tree = new BinaryTree();
        tree.root = new Node(20);
        tree.root.left = new Node(9);
        tree.root.right = new Node(49);
        tree.root.left.right = new Node(12);
        tree.root.left.left = new Node(5);
        tree.root.right.left = new Node(23);
        tree.root.right.right = new Node(52);
        tree.root.left.right.right = new Node(12);
        tree.root.right.right.left = new Node(50);
 
        Console.WriteLine("The sum of leaves is " + tree.leftLeavesSum(tree.root));
    }
}
 
  // This code is contributed by Shrikant13

Javascript

<script>
// javascript program to find sum of all left leaves
class Node {
    constructor(item) {
        this.data = item;
        this.left = this.right = null;
    }
}
 
// Passing sum as accumulator and implementing pass by reference
// of sum variable
class Sum {
constructor(){
    this.sum = 0;
    }
}
 
 
    var root;
 
    /* Pass in a sum variable as an accumulator */
    function leftLeavesSumRec( node,  isleft,  summ) {
        if (node == null)
            return;
 
        // Check whether this node is a leaf node and is left.
        if (node.left == null && node.right == null && isleft)
            summ.sum = summ.sum + node.data;
 
        // Pass true for left and false for right
        leftLeavesSumRec(node.left, true, summ);
        leftLeavesSumRec(node.right, false, summ);
    }
 
    // A wrapper over above recursive function
    function leftLeavesSum( node) {
         suum = new Sum();
 
        // use the above recursive function to evaluate sum
        leftLeavesSumRec(node, false, suum);
 
        return suum.sum;
    }
 
    // Driver program
     
         
        root = new Node(20);
        root.left = new Node(9);
        root.right = new Node(49);
        root.left.right = new Node(12);
        root.left.left = new Node(5);
        root.right.left = new Node(23);
        root.right.right = new Node(52);
        root.left.right.right = new Node(12);
        root.right.right.left = new Node(50);
 
        document.write("The sum of leaves is " + leftLeavesSum(root));
 
// This code contributed by gauravrajput1
</script>

Output

Sum of left leaves is 78

Following is Another Method to solve the above problem. We can pass bool as parameter in the function to check if it is a left or right node. Time complexity of this method is also O(n).

C

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
 
struct Node {
    int data;
    struct Node* left;
    struct Node* right;
};
 
typedef struct Node str_node;
 
str_node* create(int item);
int sumAllLeaftLeaves(str_node* node, bool isLeft);
 
int main(void)
{
    int d = 0;
    str_node* root = create(20);
    root->left = create(9);
    root->right = create(49);
    root->right->left = create(23);
    root->right->right = create(52);
    root->right->right->left = create(50);
    root->left->left = create(5);
    root->left->right = create(12);
    root->left->right->right = create(12);
 
    printf("\nSum of left leaves is: %d ",
           sumAllLeaftLeaves(root, false));
 
    return 0;
}
 
str_node* create(int item)
{
    str_node* newnode = (str_node*)malloc(sizeof(str_node));
    newnode->data = item;
    newnode->left = NULL;
    newnode->right = NULL;
    return newnode;
}
 
int sumAllLeaftLeaves(str_node* node, bool isLeft)
{
    // base case:
    if (node == NULL)
        return 0;
    // check whether this node is a leaf node and is left.
    if (node->left == NULL && node->right == NULL && isLeft)
        return node->data;
 
    // recursive case
    return sumAllLeaftLeaves(node->left, true)
           + sumAllLeaftLeaves(node->right, false);
}

Output:

Sum of left leaves is 78

Iterative Approach :
This is the Iterative Way to find the sum of the left leaves.
Idea is to perform Depth-First Traversal on the tree (either Inorder, Preorder or Postorder) using a stack and checking if the Left Child is a Leaf node. If it is, then add the nodes value to the sum variable

C++

// C++ program to find sum of all left leaves
#include<bits/stdc++.h>
using namespace std;
 
// A binary tree node
class Node
{
    public:
    int key;
    Node* left, *right;
     
    // A constructor to create a new Node
    Node(int key_)
    {
        key = key_;
        left = NULL;
        right = NULL;
    }
};
 
// Return the sum of left leaf nodes
int sumOfLeftLeaves(Node* root)
{
    if(root == NULL)
        return 0;
     
    // Using a stack_ for Depth-First
    // Traversal of the tree
    stack<Node*> stack_;
    stack_.push(root);
     
    // sum holds the sum of all the left leaves
    int sum = 0;
 
    while(stack_.size() > 0)
    {
        Node* currentNode = stack_.top();
        stack_.pop();
 
        if (currentNode->left != NULL)
        {
            stack_.push(currentNode->left);
             
            // Check if currentNode's left
            // child is a leaf node
            if(currentNode->left->left == NULL &&
               currentNode->left->right == NULL)
            {
                // if currentNode is a leaf,
                // add its data to the sum
                sum = sum + currentNode->left->key ;
            }
        }
        if (currentNode->right != NULL)
            stack_.push(currentNode->right);
    }
         
    return sum;
}
 
// Driver Code
int main()
{
    Node *root = new Node(20);
    root->left= new Node(9);
    root->right = new Node(49);
    root->right->left = new Node(23);
    root->right->right= new Node(52);
    root->right->right->left = new Node(50);
    root->left->left = new Node(5);
    root->left->right = new Node(12);
    root->left->right->right = new Node(12);
     
    cout << "Sum of left leaves is "
      << sumOfLeftLeaves(root) << endl;
    return 0;
}
 
// This code is contributed by Arnab Kundu

Java

// Java program to find sum of all left leaves
import java.util.*;
class GFG
{
 
// A binary tree node
static class Node
{
    int key;
    Node left, right;
     
    // A constructor to create a new Node
    Node(int key_)
    {
        this.key = key_;
        this.left = null;
        this.right = null;
    }
};
 
// Return the sum of left leaf nodes
static int sumOfLeftLeaves(Node root)
{
    if(root == null)
    return 0;
     
    // Using a stack_ for Depth-First
    // Traversal of the tree
    Stack<Node> stack_ = new Stack<>();
    stack_.push(root);
     
    // sum holds the sum of all the left leaves
    int sum = 0;
 
    while(stack_.size() > 0)
    {
        Node currentNode = stack_.peek();
        stack_.pop();
 
        if (currentNode.left != null)
        {
            stack_.add(currentNode.left);
             
            // Check if currentNode's left
            // child is a leaf node
            if(currentNode.left.left == null &&
               currentNode.left.right == null)
            {
                // if currentNode is a leaf,
                // add its data to the sum
                sum = sum + currentNode.left.key ;
            }
        }
        if (currentNode.right != null)
            stack_.add(currentNode.right);
    }      
    return sum;
}
 
// Driver Code
public static void main(String[] args)
{
    Node root = new Node(20);
    root.left= new Node(9);
    root.right = new Node(49);
    root.right.left = new Node(23);
    root.right.right= new Node(52);
    root.right.right.left = new Node(50);
    root.left.left = new Node(5);
    root.left.right = new Node(12);
    root.left.right.right = new Node(12);
     
    System.out.print("Sum of left leaves is "
      + sumOfLeftLeaves(root) +"\n");
}
}
 
// This code is contributed by aashish1995

Python3

# Python3 program to find sum of all left leaves
 
# A binary tree node
class Node:
 
    # A constructor to create a new Node
    def __init__(self, key):
        self.key = key
        self.left = None
        self.right = None
 
 
# Return the sum of left leaf nodes
def sumOfLeftLeaves(root):
    if(root is None):
        return
     
    # Using a stack for Depth-First Traversal of the tree
    stack = []  
    stack.append(root)
     
    # sum holds the sum of all the left leaves
    sum = 0
 
    while len(stack) > 0:
        currentNode = stack.pop()
        if currentNode.left is not None:
            stack.append(currentNode.left)
             
            # Check if currentNode's left child is a leaf node
            if currentNode.left.left is None and currentNode.left.right is None:
 
                # if currentNode is a leaf, add its data to the sum 
                sum = sum + currentNode.left.data
 
        if currentNode.right is not None:
            stack.append(currentNode.right)
    return sum
 
# Driver Code
root = Tree(20);
root.left= Tree(9);
root.right   = Tree(49);
root.right.left = Tree(23);
root.right.right= Tree(52);
root.right.right.left  = Tree(50);
root.left.left  = Tree(5);
root.left.right = Tree(12);
root.left.right.right  = Tree(12);
 
print('Sum of left leaves is {}'.format(sumOfLeftLeaves(root)))

C#

// C# program to find sum of all left leaves
using System;
using System.Collections.Generic;
class GFG
{
 
  // A binary tree node
  public
    class Node
    {
      public
        int key;
      public
        Node left, right;
 
      // A constructor to create a new Node
      public
        Node(int key_)
      {
        this.key = key_;
        this.left = null;
        this.right = null;
      }
    };
 
  // Return the sum of left leaf nodes
  static int sumOfLeftLeaves(Node root)
  {
    if(root == null)
      return 0;
 
    // Using a stack_ for Depth-First
    // Traversal of the tree
    Stack<Node> stack_ = new Stack<Node>();
    stack_.Push(root);
 
    // sum holds the sum of all the left leaves
    int sum = 0;
    while(stack_.Count > 0)
    {
      Node currentNode = stack_.Peek();
      stack_.Pop();
      if (currentNode.left != null)
      {
        stack_.Push(currentNode.left);
 
        // Check if currentNode's left
        // child is a leaf node
        if(currentNode.left.left == null &&
           currentNode.left.right == null)
        {
 
          // if currentNode is a leaf,
          // add its data to the sum
          sum = sum + currentNode.left.key ;
        }
      }
      if (currentNode.right != null)
        stack_.Push(currentNode.right);
    }      
    return sum;
  }
 
  // Driver Code
  public static void Main(String[] args)
  {
    Node root = new Node(20);
    root.left= new Node(9);
    root.right = new Node(49);
    root.right.left = new Node(23);
    root.right.right= new Node(52);
    root.right.right.left = new Node(50);
    root.left.left = new Node(5);
    root.left.right = new Node(12);
    root.left.right.right = new Node(12);
 
    Console.Write("Sum of left leaves is "
                  + sumOfLeftLeaves(root) +"\n");
  }
}
 
// This code is contributed by Rajput-Ji

Javascript

<script>
  
 
// JavaScript program to find sum of all left leaves
// A binary tree node
class Node
{
 
  // A constructor to create a new Node
  constructor(key_)
  {
    this.key = key_;
    this.left = null;
    this.right = null;
  }
};
// Return the sum of left leaf nodes
function sumOfLeftLeaves(root)
{
  if(root == null)
    return 0;
  // Using a stack_ for Depth-First
  // Traversal of the tree
  var stack_ = [];
  stack_.push(root);
  // sum holds the sum of all the left leaves
  var sum = 0;
  while(stack_.length > 0)
  {
    var currentNode = stack_[stack_.length-1];
    stack_.pop();
    if (currentNode.left != null)
    {
      stack_.push(currentNode.left);
      // Check if currentNode's left
      // child is a leaf node
      if(currentNode.left.left == null &&
         currentNode.left.right == null)
      {
        // if currentNode is a leaf,
        // add its data to the sum
        sum = sum + currentNode.left.key ;
      }
    }
    if (currentNode.right != null)
      stack_.push(currentNode.right);
  }      
  return sum;
}
 
// Driver Code
var root = new Node(20);
root.left= new Node(9);
root.right = new Node(49);
root.right.left = new Node(23);
root.right.right= new Node(52);
root.right.right.left = new Node(50);
root.left.left = new Node(5);
root.left.right = new Node(12);
root.left.right.right = new Node(12);
document.write("Sum of left leaves is "
              + sumOfLeftLeaves(root) +"<br>");
 
 
</script>

Output

Sum of left leaves is 78

Thanks to Shubham Tambere for suggesting this approach.

BFS Approach: We can do BFS traversal and keep a separate variable for denoting if it is a left child or right child of a node. As soon as we encounter a leaf, we check if it is a left child of its parent or right child of its parent. If it is a left child, we add its value in the sum.

Below is the implementation of the above approach:

C++

// C++ program to find sum of all left leaves
#include <bits/stdc++.h>
using namespace std;
 
// A binary tree node
class Node {
public:
    int key;
    Node *left, *right;
 
    // constructor to create a new Node
    Node(int key_)
    {
        key = key_;
        left = NULL;
        right = NULL;
    }
};
 
// Return the sum of left leaf nodes
int sumOfLeftLeaves(Node* root)
{
    if (root == NULL)
        return 0;
    // A queue of pairs to do bfs traversal
    // and keep track if the node is a left
    // or right child if boolean value
    // is true then it is a left child.
    queue<pair<Node*, bool> > q;
    q.push({ root, 0 });
    int sum = 0;
    // do bfs traversal
    while (!q.empty()) {
        Node* temp = q.front().first;
        bool is_left_child =
                   q.front().second;
        q.pop();
        // if temp is a leaf node and
        // left child of its parent
        if (!temp->left && !temp->right &&
                            is_left_child)
            sum = sum + temp->key;
        // if it is not leaf then
        // push its children nodes
        // into queue
        if (temp->left) {
            // boolean value is true
            // here because it is left
            // child of its parent
            q.push({ temp->left, 1 });
        }
        if (temp->right) {
            // boolean value is false
            // here because it is
            // right child of its parent
            q.push({ temp->right, 0 });
        }
    }
    return sum;
}
 
// Driver Code
int main()
{
    Node* root = new Node(20);
    root->left = new Node(9);
    root->right = new Node(49);
    root->right->left = new Node(23);
    root->right->right = new Node(52);
    root->right->right->left = new Node(50);
    root->left->left = new Node(5);
    root->left->right = new Node(12);
    root->left->right->right = new Node(12);
 
    cout << "Sum of left leaves is "
         << sumOfLeftLeaves(root) << endl;
    return 0;
}

Java

// Java program to find sum of all left leaves
import java.util.*;
class GFG
{
 
// A binary tree node
static class Node
{
    int key;
    Node left, right;
 
    // constructor to create a new Node
    Node(int key_)
    {
        key = key_;
        left = null;
        right = null;
    }
};
static class pair
{
    Node first;
    boolean second;
    public pair(Node first, boolean second) 
    {
        this.first = first;
        this.second = second;
    }   
}
   
// Return the sum of left leaf nodes
static int sumOfLeftLeaves(Node root)
{
    if (root == null)
    return 0;
   
    // A queue of pairs to do bfs traversal
    // and keep track if the node is a left
    // or right child if boolean value
    // is true then it is a left child.
    Queue<pair > q = new LinkedList<>();
    q.add(new pair( root, false ));
    int sum = 0;
   
    // do bfs traversal
    while (!q.isEmpty())
    {
        Node temp = q.peek().first;
        boolean is_left_child =
                   q.peek().second;
        q.remove();
       
        // if temp is a leaf node and
        // left child of its parent
        if (is_left_child)
            sum = sum + temp.key;
        if(temp.left != null && temp.right != null && is_left_child)
            sum = sum-temp.key;
       
        // if it is not leaf then
        // push its children nodes
        // into queue
        if (temp.left != null)
        {
           
            // boolean value is true
            // here because it is left
            // child of its parent
            q.add(new pair( temp.left, true));
        }
        if (temp.right != null)
        {
           
            // boolean value is false
            // here because it is
            // right child of its parent
            q.add(new pair( temp.right, false));
        }
    }
    return sum;
}
 
// Driver Code
public static void main(String[] args)
{
    Node root = new Node(20);
    root.left = new Node(9);
    root.right = new Node(49);
    root.right.left = new Node(23);
    root.right.right = new Node(52);
    root.right.right.left = new Node(50);
    root.left.left = new Node(5);
    root.left.right = new Node(12);
    root.left.right.right = new Node(12);
 
    System.out.print("Sum of left leaves is "
         + sumOfLeftLeaves(root) +"\n");
}
}
 
// This code is contributed by gauravrajput1.

Python3

# Python3 program to find sum of
# all left leaves
from collections import deque
 
# A binary tree node
class Node:
     
    def __init__(self, x):
         
        self.key = x
        self.left = None
        self.right = None
 
# Return the sum of left leaf nodes
def sumOfLeftLeaves(root):
     
    if (root == None):
        return 0
         
    # A queue of pairs to do bfs traversal
    # and keep track if the node is a left
    # or right child if boolean value
    # is true then it is a left child.
    q = deque()
    q.append([root, 0])
    sum = 0
     
    # Do bfs traversal
    while (len(q) > 0):
        temp = q[0][0]
        is_left_child = q[0][1]
        q.popleft()
         
        # If temp is a leaf node and
        # left child of its parent
        if (not temp.left and
            not temp.right and
            is_left_child):
            sum = sum + temp.key
             
        # If it is not leaf then
        # push its children nodes
        # into queue
        if (temp.left):
             
            # Boolean value is true
            # here because it is left
            # child of its parent
            q.append([temp.left, 1])
             
        if (temp.right):
             
            # Boolean value is false
            # here because it is
            # right child of its parent
            q.append([temp.right, 0])
 
    return sum
 
# Driver Code
if __name__ == '__main__':
     
    root = Node(20)
    root.left = Node(9)
    root.right = Node(49)
    root.right.left = Node(23)
    root.right.right = Node(52)
    root.right.right.left = Node(50)
    root.left.left = Node(5)
    root.left.right = Node(12)
    root.left.right.right = Node(12)
 
    print("Sum of left leaves is",
          sumOfLeftLeaves(root))
 
# This code is contributed by mohit kumar 29

C#

// C# program to find sum of all left leaves
using System;
using System.Collections.Generic;
public class GFG
{
 
  // A binary tree node
  public
 
    class Node
    {
      public int key;
      public
 
        Node left,
      right;
 
      // constructor to create a new Node
      public
 
        Node(int key_)
      {
        key = key_;
        left = null;
        right = null;
      }
    };
  public
 
    class pair {
      public
 
        Node first;
      public
 
        bool second;
      public pair(Node first, bool second)
      {
        this.first = first;
        this.second = second;
      }
    }
 
  // Return the sum of left leaf nodes
  static int sumOfLeftLeaves(Node root)
  {
    if (root == null)
      return 0;
 
    // A queue of pairs to do bfs traversal
    // and keep track if the node is a left
    // or right child if bool value
    // is true then it is a left child.
    Queue<pair> q = new Queue<pair>();
    q.Enqueue(new pair(root, false));
    int sum = 0;
 
    // do bfs traversal
    while (q.Count != 0)
    {
      Node temp = q.Peek().first;
      bool is_left_child = q.Peek().second;
      q.Dequeue();
 
      // if temp is a leaf node and
      // left child of its parent
      if (is_left_child)
        sum = sum + temp.key;
      if (temp.left != null && temp.right != null
          && is_left_child)
        sum = sum - temp.key;
 
      // if it is not leaf then
      // push its children nodes
      // into queue
      if (temp.left != null)
      {
 
        // bool value is true
        // here because it is left
        // child of its parent
        q.Enqueue(new pair(temp.left, true));
      }
      if (temp.right != null)
      {
 
        // bool value is false
        // here because it is
        // right child of its parent
        q.Enqueue(new pair(temp.right, false));
      }
    }
    return sum;
  }
 
  // Driver Code
  public static void Main(String[] args)
  {
    Node root = new Node(20);
    root.left = new Node(9);
    root.right = new Node(49);
    root.right.left = new Node(23);
    root.right.right = new Node(52);
    root.right.right.left = new Node(50);
    root.left.left = new Node(5);
    root.left.right = new Node(12);
    root.left.right.right = new Node(12);
 
    Console.Write("Sum of left leaves is "
                  + sumOfLeftLeaves(root) + "\n");
  }
}
 
// This code is contributed by gauravrajput1

Javascript

<script>
 
    // JavaScript program to find sum of all left leaves
     
    class Node
    {
        constructor(key_) {
           this.left = null;
           this.right = null;
           this.key = key_;
        }
    }
   
    // Return the sum of left leaf nodes
    function sumOfLeftLeaves(root)
    {
        if (root == null)
            return 0;
 
        // A queue of pairs to do bfs traversal
        // and keep track if the node is a left
        // or right child if boolean value
        // is true then it is a left child.
        let q = [];
        q.push([root, false ]);
        let sum = 0;
 
        // do bfs traversal
        while (q.length > 0)
        {
            let temp = q[0][0];
            let is_left_child =
                       q[0][1];
            q.shift();
 
            // if temp is a leaf node and
            // left child of its parent
            if (is_left_child)
                sum = sum + temp.key;
            if(temp.left != null &&
            temp.right != null && is_left_child)
                sum = sum-temp.key;
 
            // if it is not leaf then
            // push its children nodes
            // into queue
            if (temp.left != null)
            {
 
                // boolean value is true
                // here because it is left
                // child of its parent
                q.push([temp.left, true]);
            }
            if (temp.right != null)
            {
 
                // boolean value is false
                // here because it is
                // right child of its parent
                q.push([temp.right, false]);
            }
        }
        return sum;
    }
     
    let root = new Node(20);
    root.left = new Node(9);
    root.right = new Node(49);
    root.right.left = new Node(23);
    root.right.right = new Node(52);
    root.right.right.left = new Node(50);
    root.left.left = new Node(5);
    root.left.right = new Node(12);
    root.left.right.right = new Node(12);
  
    document.write("Sum of left leaves is "
         + sumOfLeftLeaves(root) +"</br>");
     
</script>

Output

Sum of left leaves is 78

Time Complexity: O(N)
Auxiliary Space: O(N)

This article is contributed by Manish. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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