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Level Order Predecessor of a node in Binary Tree

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  • Difficulty Level : Medium
  • Last Updated : 14 Jun, 2022

Given a binary tree and a node in the binary tree, find Levelorder Predecessor of the given node. That is, the node that appears before the given node in the level order traversal of the tree.
Note: The task is not just to print the data of the node, you have to return the complete node from the tree.
Examples
 

Consider the following binary tree
              20            
           /      \         
          10       26       
         /  \     /   \     
       4     18  24    27   
            /  \
           14   19
          /  \
         13  15

Levelorder traversal of given tree is:
20, 10, 26, 4, 18, 24, 27, 14, 19, 13, 15

Input : 19
Output : 14

Input : 4
Output : 26

 

Approach
 

  1. Check if the root is NULL, that is tree is empty. If true then return NULL.
  2. Check if the given node is root. If true than there will be no predecessor of root, so return NULL.
  3. Otherwise, perform Level Order Traversal on the tree using a Queue and a temporary pointer prev to maintain the last node during the traversal.
  4. At every step of the level order traversal, check if the current node matches with the given node.
  5. If True, stop traversing any further and return the node stored in the pointer prev as it is the node accessed just before the current node during the traversal.

Below is the implementation of the above approach: 
 

C++




// CPP program to find Levelorder
// Predecessor of given node in the
// Binary Tree
 
#include <bits/stdc++.h>
using namespace std;
 
// Tree Node
struct Node {
    struct Node *left, *right;
    int value;
};
 
// Utility function to create a
// new node with given value
struct Node* newNode(int value)
{
    Node* temp = new Node;
    temp->left = temp->right = NULL;
    temp->value = value;
 
    return temp;
}
 
// Function to find the Level Order Predecessor
// of a given Node in Binary Tree
Node* levelOrderPredecessor(Node* root, Node* key)
{
    // Base Case
    if (root == NULL)
        return NULL;
 
    // If root equals to key
    if (root == key) {
 
        // There is no Predecessor of
        // root node
        return NULL;
    }
 
    // Create an empty queue for level
    // order traversal
    queue<Node*> q;
 
    // Enqueue Root
    q.push(root);
 
    // Temporary node to keep track of the
    // last node
    Node* prev = NULL;
 
    while (!q.empty()) {
        Node* nd = q.front();
        q.pop();
 
        if (nd == key)
            break;
        else
            prev = nd;
 
        if (nd->left != NULL) {
            q.push(nd->left);
        }
 
        if (nd->right != NULL) {
            q.push(nd->right);
        }
    }
 
    return prev;
}
 
// Driver code
int main()
{
    struct Node* root = newNode(20);
    root->left = newNode(10);
    root->left->left = newNode(4);
    root->left->right = newNode(18);
    root->right = newNode(26);
    root->right->left = newNode(24);
    root->right->right = newNode(27);
    root->left->right->left = newNode(14);
    root->left->right->left->left = newNode(13);
    root->left->right->left->right = newNode(15);
    root->left->right->right = newNode(19);
 
    struct Node* key = root->left->right->right;
 
    struct Node* res = levelOrderPredecessor(root, key);
 
    if (res)
        cout << "LevelOrder Predecessor of " << key->value
             << " is " << res->value;
    else
        cout << "LevelOrder Predecessor of " << key->value
             << " is "
             << "NULL";
 
    return 0;
}

Java




// Java program to find Levelorder
// Predecessor of given node in the
// Binary Tree
import java.util.*;
class GfG {
 
// Tree Node
static class Node {
    Node left, right;
    int value;
}
 
// Utility function to create a
// new node with given value
static Node newNode(int value)
{
    Node temp = new Node();
    temp.left = null;
    temp.right = null;
    temp.value = value;
 
    return temp;
}
 
// Function to find the Level Order Predecessor
// of a given Node in Binary Tree
static Node levelOrderPredecessor(Node root, Node key)
{
    // Base Case
    if (root == null)
        return null;
 
    // If root equals to key
    if (root == key) {
 
        // There is no Predecessor of
        // root node
        return null;
    }
 
    // Create an empty queue for level
    // order traversal
    Queue<Node> q = new LinkedList<Node> ();
 
    // Enqueue Root
    q.add(root);
 
    // Temporary node to keep track of the
    // last node
    Node prev = null;
 
    while (!q.isEmpty()) {
        Node nd = q.peek();
        q.remove();
 
        if (nd == key)
            break;
        else
            prev = nd;
 
        if (nd.left != null) {
            q.add(nd.left);
        }
 
        if (nd.right != null) {
            q.add(nd.right);
        }
    }
 
    return prev;
}
 
// Driver code
public static void main(String[] args)
{
    Node root = newNode(20);
    root.left = newNode(10);
    root.left.left = newNode(4);
    root.left.right = newNode(18);
    root.right = newNode(26);
    root.right.left = newNode(24);
    root.right.right = newNode(27);
    root.left.right.left = newNode(14);
    root.left.right.left.left = newNode(13);
    root.left.right.left.right = newNode(15);
    root.left.right.right = newNode(19);
 
    Node key = root.left.right.right;
 
    Node res = levelOrderPredecessor(root, key);
 
    if (res != null)
        System.out.println("LevelOrder Predecessor of " + key.value + " is " + res.value);
    else
        System.out.println("LevelOrder Predecessor of " + key.value+ " is null");
 
}
}

Python3




"""Python3 program to find Level order
Predecessor of given node in the
Binary Tree"""
 
# A Binary Tree Node
# Utility function to create a
# new tree node
class newNode:
 
    # Constructor to create a newNode
    def __init__(self, data):
        self.value = data
        self.left = None
        self.right = self.parent = None
 
# Function to find the Level Order Predecessor
# of a given Node in Binary Tree
def levelOrderPredecessor(root, key) :
 
    # Base Case
    if (root == None) :
        return None
 
    # If root equals to key
    if (root == key):
         
        # There is no Predecessor of
        # root node
        return None
     
    # Create an empty queue for level
    # order traversal
    q = []
 
    # Enqueue Root
    q.append(root)
 
    # Temporary node to keep track
    # of the last node
    prev = None
 
    while (len(q)):
        nd = q[0]
        q.pop(0)
 
        if (nd == key) :
            break
        else:
            prev = nd
 
        if (nd.left != None):
            q.append(nd.left)
         
        if (nd.right != None):
            q.append(nd.right)
    return prev
 
# Driver Code
if __name__ == '__main__':
 
    root = newNode(20)
    root.left = newNode(10)
    root.left.left = newNode(4)
    root.left.right = newNode(18)
    root.right = newNode(26)
    root.right.left = newNode(24)
    root.right.right = newNode(27)
    root.left.right.left = newNode(14)
    root.left.right.left.left = newNode(13)
    root.left.right.left.right = newNode(15)
    root.left.right.right = newNode(19)
 
    key = root.left.right.right
 
    res = levelOrderPredecessor(root, key)
 
    if (res) :
        print("LevelOrder Predecessor of",
               key.value, "is", res.value)
    else:
        print("LevelOrder Predecessor of",
                  key.value, "is", "None")
 
# This code is contributed by
# SHUBHAMSINGH10

C#




// C# program to find Levelorder
// Predecessor of given node in the
// Binary Tree
using System;
using System.Collections.Generic;
 
class GfG
{
 
    // Tree Node
    class Node
    {
        public Node left, right;
        public int value;
    }
 
    // Utility function to create a
    // new node with given value
    static Node newNode(int value)
    {
        Node temp = new Node();
        temp.left = null;
        temp.right = null;
        temp.value = value;
 
        return temp;
    }
 
    // Function to find the Level Order Predecessor
    // of a given Node in Binary Tree
    static Node levelOrderPredecessor(Node root, Node key)
    {
        // Base Case
        if (root == null)
            return null;
 
        // If root equals to key
        if (root == key)
        {
 
            // There is no Predecessor of
            // root node
            return null;
        }
 
        // Create an empty queue for level
        // order traversal
        Queue<Node> q = new Queue<Node> ();
 
        // Enqueue Root
        q.Enqueue(root);
 
        // Temporary node to keep track of the
        // last node
        Node prev = null;
 
        while (q.Count!=0)
        {
            Node nd = q.Peek();
            q.Dequeue();
 
            if (nd == key)
                break;
            else
                prev = nd;
 
            if (nd.left != null)
            {
                q.Enqueue(nd.left);
            }
 
            if (nd.right != null)
            {
                q.Enqueue(nd.right);
            }
        }
 
        return prev;
    }
 
    // Driver code
    public static void Main(String[] args)
    {
        Node root = newNode(20);
        root.left = newNode(10);
        root.left.left = newNode(4);
        root.left.right = newNode(18);
        root.right = newNode(26);
        root.right.left = newNode(24);
        root.right.right = newNode(27);
        root.left.right.left = newNode(14);
        root.left.right.left.left = newNode(13);
        root.left.right.left.right = newNode(15);
        root.left.right.right = newNode(19);
 
        Node key = root.left.right.right;
 
        Node res = levelOrderPredecessor(root, key);
 
        if (res != null)
            Console.WriteLine("LevelOrder Predecessor of " +
                                key.value + " is " + res.value);
        else
            Console.WriteLine("LevelOrder Predecessor of " +
                                key.value+ " is null");
    }
}
 
// This code is contributed by PrinciRaj1992

Javascript




<script>
 
    // JavaScript program to find Levelorder
    // Predecessor of given node in the
    // Binary Tree 
     
    // Tree Node
    class Node
    {
        constructor(value) {
           this.left = null;
           this.right = null;
           this.value = value;
        }
    }
     
    // Utility function to create a
    // new node with given value
    function newNode(value)
    {
        let temp = new Node(value);
        return temp;
    }
 
    // Function to find the Level Order Predecessor
    // of a given Node in Binary Tree
    function levelOrderPredecessor(root, key)
    {
        // Base Case
        if (root == null)
            return null;
 
        // If root equals to key
        if (root == key) {
 
            // There is no Predecessor of
            // root node
            return null;
        }
 
        // Create an empty queue for level
        // order traversal
        let q = [];
 
        // Enqueue Root
        q.push(root);
 
        // Temporary node to keep track of the
        // last node
        let prev = null;
 
        while (q.length > 0) {
            let nd = q[0];
            q.shift();
 
            if (nd == key)
                break;
            else
                prev = nd;
 
            if (nd.left != null) {
                q.push(nd.left);
            }
 
            if (nd.right != null) {
                q.push(nd.right);
            }
        }
 
        return prev;
    }
     
    let root = newNode(20);
    root.left = newNode(10);
    root.left.left = newNode(4);
    root.left.right = newNode(18);
    root.right = newNode(26);
    root.right.left = newNode(24);
    root.right.right = newNode(27);
    root.left.right.left = newNode(14);
    root.left.right.left.left = newNode(13);
    root.left.right.left.right = newNode(15);
    root.left.right.right = newNode(19);
   
    let key = root.left.right.right;
   
    let res = levelOrderPredecessor(root, key);
   
    if (res != null)
        document.write("LevelOrder Predecessor of " +
        key.value + " is " + res.value);
    else
        document.write("LevelOrder Predecessor of " +
        key.value+ " is null");
 
</script>

Output: 

LevelOrder Predecessor of 19 is 14

 

Time Complexity: O(N), as we are using a while loop which will traverse N times, where N is the number of nodes in the tree.

Auxiliary Space: O(N), as we are using extra space for the queue, which we are using for the level order traversal.


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