# Level Order Binary Tree Traversal

• Difficulty Level : Easy
• Last Updated : 16 Jun, 2022

Level order traversal of a tree is breadth first traversal for the tree. Level order traversal of the above tree is 1 2 3 4 5

Method 1 (Use function to print a current level)

Algorithm:
There are basically two functions in this method. One is to print all nodes at a given level (printCurrentLevel), and the other is to print the level order traversal of the tree (printLevelorder). printLevelorder makes use of printCurrentLevel to print nodes at all levels one by one starting from the root.

/*Function to print level order traversal of tree*/
printLevelorder(tree)
for d = 1 to height(tree)
printCurrentLevel(tree, d);

/*Function to print all nodes at a current level*/
printCurrentLevel(tree, level)
if tree is NULL then return;
if level is 1, then
print(tree->data);
else if level greater than 1, then
printCurrentLevel(tree->left, level-1);
printCurrentLevel(tree->right, level-1);

Implementation:

## C++

 `// Recursive CPP program for level``// order traversal of Binary Tree``#include ``using` `namespace` `std;` `/* A binary tree node has data,``pointer to left child``and a pointer to right child */``class` `node {``public``:``    ``int` `data;``    ``node *left, *right;``};` `/* Function prototypes */``void` `printCurrentLevel(node* root, ``int` `level);``int` `height(node* node);``node* newNode(``int` `data);` `/* Function to print level``order traversal a tree*/``void` `printLevelOrder(node* root)``{``    ``int` `h = height(root);``    ``int` `i;``    ``for` `(i = 1; i <= h; i++)``        ``printCurrentLevel(root, i);``}` `/* Print nodes at a current level */``void` `printCurrentLevel(node* root, ``int` `level)``{``    ``if` `(root == NULL)``        ``return``;``    ``if` `(level == 1)``        ``cout << root->data << ``" "``;``    ``else` `if` `(level > 1) {``        ``printCurrentLevel(root->left, level - 1);``        ``printCurrentLevel(root->right, level - 1);``    ``}``}` `/* Compute the "height" of a tree -- the number of``    ``nodes along the longest path from the root node``    ``down to the farthest leaf node.*/``int` `height(node* node)``{``    ``if` `(node == NULL)``        ``return` `0;``    ``else` `{``        ``/* compute the height of each subtree */``        ``int` `lheight = height(node->left);``        ``int` `rheight = height(node->right);` `        ``/* use the larger one */``        ``if` `(lheight > rheight) {``            ``return` `(lheight + 1);``        ``}``        ``else` `{``            ``return` `(rheight + 1);``        ``}``    ``}``}` `/* Helper function that allocates``a new node with the given data and``NULL left and right pointers. */``node* newNode(``int` `data)``{``    ``node* Node = ``new` `node();``    ``Node->data = data;``    ``Node->left = NULL;``    ``Node->right = NULL;` `    ``return` `(Node);``}` `/* Driver code*/``int` `main()``{``    ``node* root = newNode(1);``    ``root->left = newNode(2);``    ``root->right = newNode(3);``    ``root->left->left = newNode(4);``    ``root->left->right = newNode(5);` `    ``cout << ``"Level Order traversal of binary tree is \n"``;``    ``printLevelOrder(root);` `    ``return` `0;``}` `// This code is contributed by rathbhupendra`

## C

 `// Recursive C program for level``// order traversal of Binary Tree``#include ``#include ` `/* A binary tree node has data,``   ``pointer to left child``   ``and a pointer to right child */``struct` `node {``    ``int` `data;``    ``struct` `node *left, *right;``};` `/* Function prototypes */``void` `printCurrentLevel(``struct` `node* root, ``int` `level);``int` `height(``struct` `node* node);``struct` `node* newNode(``int` `data);` `/* Function to print level order traversal a tree*/``void` `printLevelOrder(``struct` `node* root)``{``    ``int` `h = height(root);``    ``int` `i;``    ``for` `(i = 1; i <= h; i++)``        ``printCurrentLevel(root, i);``}` `/* Print nodes at a current level */``void` `printCurrentLevel(``struct` `node* root, ``int` `level)``{``    ``if` `(root == NULL)``        ``return``;``    ``if` `(level == 1)``        ``printf``(``"%d "``, root->data);``    ``else` `if` `(level > 1) {``        ``printCurrentLevel(root->left, level - 1);``        ``printCurrentLevel(root->right, level - 1);``    ``}``}` `/* Compute the "height" of a tree -- the number of``    ``nodes along the longest path from the root node``    ``down to the farthest leaf node.*/``int` `height(``struct` `node* node)``{``    ``if` `(node == NULL)``        ``return` `0;``    ``else` `{``        ``/* compute the height of each subtree */``        ``int` `lheight = height(node->left);``        ``int` `rheight = height(node->right);` `        ``/* use the larger one */``        ``if` `(lheight > rheight)``            ``return` `(lheight + 1);``        ``else``            ``return` `(rheight + 1);``    ``}``}` `/* Helper function that allocates a new node with the``   ``given data and NULL left and right pointers. */``struct` `node* newNode(``int` `data)``{``    ``struct` `node* node``        ``= (``struct` `node*)``malloc``(``sizeof``(``struct` `node));``    ``node->data = data;``    ``node->left = NULL;``    ``node->right = NULL;` `    ``return` `(node);``}` `/* Driver program to test above functions*/``int` `main()``{``    ``struct` `node* root = newNode(1);``    ``root->left = newNode(2);``    ``root->right = newNode(3);``    ``root->left->left = newNode(4);``    ``root->left->right = newNode(5);` `    ``printf``(``"Level Order traversal of binary tree is \n"``);``    ``printLevelOrder(root);` `    ``return` `0;``}`

## Java

 `// Recursive Java program for level``// order traversal of Binary Tree` `/* Class containing left and right child of current``   ``node and key value*/``class` `Node {``    ``int` `data;``    ``Node left, right;``    ``public` `Node(``int` `item)``    ``{``        ``data = item;``        ``left = right = ``null``;``    ``}``}` `class` `BinaryTree {``    ``// Root of the Binary Tree``    ``Node root;` `    ``public` `BinaryTree() { root = ``null``; }` `    ``/* function to print level order traversal of tree*/``    ``void` `printLevelOrder()``    ``{``        ``int` `h = height(root);``        ``int` `i;``        ``for` `(i = ``1``; i <= h; i++)``            ``printCurrentLevel(root, i);``    ``}` `    ``/* Compute the "height" of a tree -- the number of``    ``nodes along the longest path from the root node``    ``down to the farthest leaf node.*/``    ``int` `height(Node root)``    ``{``        ``if` `(root == ``null``)``            ``return` `0``;``        ``else` `{``            ``/* compute  height of each subtree */``            ``int` `lheight = height(root.left);``            ``int` `rheight = height(root.right);` `            ``/* use the larger one */``            ``if` `(lheight > rheight)``                ``return` `(lheight + ``1``);``            ``else``                ``return` `(rheight + ``1``);``        ``}``    ``}` `    ``/* Print nodes at the current level */``    ``void` `printCurrentLevel(Node root, ``int` `level)``    ``{``        ``if` `(root == ``null``)``            ``return``;``        ``if` `(level == ``1``)``            ``System.out.print(root.data + ``" "``);``        ``else` `if` `(level > ``1``) {``            ``printCurrentLevel(root.left, level - ``1``);``            ``printCurrentLevel(root.right, level - ``1``);``        ``}``    ``}` `    ``/* Driver program to test above functions */``    ``public` `static` `void` `main(String args[])``    ``{``        ``BinaryTree tree = ``new` `BinaryTree();``        ``tree.root = ``new` `Node(``1``);``        ``tree.root.left = ``new` `Node(``2``);``        ``tree.root.right = ``new` `Node(``3``);``        ``tree.root.left.left = ``new` `Node(``4``);``        ``tree.root.left.right = ``new` `Node(``5``);` `       ``System.out.println("Level order traversal of``                                 ``binary tree is ");``       ``tree.printLevelOrder();``    ``}``}`

## Python3

 `# Recursive Python program for level``# order traversal of Binary Tree` `# A node structure`  `class` `Node:` `    ``# A utility function to create a new node``    ``def` `__init__(``self``, key):``        ``self``.data ``=` `key``        ``self``.left ``=` `None``        ``self``.right ``=` `None`  `# Function to  print level order traversal of tree``def` `printLevelOrder(root):``    ``h ``=` `height(root)``    ``for` `i ``in` `range``(``1``, h``+``1``):``        ``printCurrentLevel(root, i)`  `# Print nodes at a current level``def` `printCurrentLevel(root, level):``    ``if` `root ``is` `None``:``        ``return``    ``if` `level ``=``=` `1``:``        ``print``(root.data, end``=``" "``)``    ``elif` `level > ``1``:``        ``printCurrentLevel(root.left, level``-``1``)``        ``printCurrentLevel(root.right, level``-``1``)`  `""" Compute the height of a tree--the number of nodes``    ``along the longest path from the root node down to``    ``the farthest leaf node``"""`  `def` `height(node):``    ``if` `node ``is` `None``:``        ``return` `0``    ``else``:``        ``# Compute the height of each subtree``        ``lheight ``=` `height(node.left)``        ``rheight ``=` `height(node.right)` `        ``# Use the larger one``        ``if` `lheight > rheight:``            ``return` `lheight``+``1``        ``else``:``            ``return` `rheight``+``1`  `# Driver program to test above function``root ``=` `Node(``1``)``root.left ``=` `Node(``2``)``root.right ``=` `Node(``3``)``root.left.left ``=` `Node(``4``)``root.left.right ``=` `Node(``5``)` `print``(``"Level order traversal of binary tree is -"``)``printLevelOrder(root)` `# This code is contributed by Nikhil Kumar Singh(nickzuck_007)`

## C#

 `// Recursive c# program for level``// order traversal of Binary Tree``using` `System;` `/* Class containing left and right``   ``child of current node and key value*/``public` `class` `Node {``    ``public` `int` `data;``    ``public` `Node left, right;``    ``public` `Node(``int` `item)``    ``{``        ``data = item;``        ``left = right = ``null``;``    ``}``}` `class` `GFG {``    ``// Root of the Binary Tree``    ``public` `Node root;` `    ``public` `void` `BinaryTree() { root = ``null``; }` `    ``/* function to print level order``       ``traversal of tree*/``    ``public` `virtual` `void` `printLevelOrder()``    ``{``        ``int` `h = height(root);``        ``int` `i;``        ``for` `(i = 1; i <= h; i++) {``            ``printCurrentLevel(root, i);``        ``}``    ``}` `    ``/* Compute the "height" of a tree --``    ``the number of nodes along the longest``    ``path from the root node down to the``    ``farthest leaf node.*/``    ``public` `virtual` `int` `height(Node root)``    ``{``        ``if` `(root == ``null``) {``            ``return` `0;``        ``}``        ``else` `{``            ``/* compute height of each subtree */``            ``int` `lheight = height(root.left);``            ``int` `rheight = height(root.right);` `            ``/* use the larger one */``            ``if` `(lheight > rheight) {``                ``return` `(lheight + 1);``            ``}``            ``else` `{``                ``return` `(rheight + 1);``            ``}``        ``}``    ``}` `    ``/* Print nodes at the current level */``    ``public` `virtual` `void` `printCurrentLevel(Node root,``                                          ``int` `level)``    ``{``        ``if` `(root == ``null``) {``            ``return``;``        ``}``        ``if` `(level == 1) {``            ``Console.Write(root.data + ``" "``);``        ``}``        ``else` `if` `(level > 1) {``            ``printCurrentLevel(root.left, level - 1);``            ``printCurrentLevel(root.right, level - 1);``        ``}``    ``}` `    ``// Driver Code``    ``public` `static` `void` `Main(``string``[] args)``    ``{``        ``GFG tree = ``new` `GFG();``        ``tree.root = ``new` `Node(1);``        ``tree.root.left = ``new` `Node(2);``        ``tree.root.right = ``new` `Node(3);``        ``tree.root.left.left = ``new` `Node(4);``        ``tree.root.left.right = ``new` `Node(5);` `        ``Console.WriteLine(``"Level order traversal "``                          ``+ ``"of binary tree is "``);``        ``tree.printLevelOrder();``    ``}``}` `// This code is contributed by Shrikant13`

## Javascript

 ``

Output

```Level Order traversal of binary tree is
1 2 3 4 5 ```

Time Complexity: O(n^2) in worst case. For a skewed tree, printGivenLevel() takes O(n) time where n is the number of nodes in the skewed tree. So time complexity of printLevelOrder() is O(n) + O(n-1) + O(n-2) + .. + O(1) which is O(n^2).
Auxiliary Space:  O(n) in the worst case. For a skewed tree, printGivenLevel() uses O(n) space for call stack. For a Balanced tree, the call stack uses O(log n) space, (i.e., the height of the balanced tree).

Method 2 (Using queue)

Algorithm: For each node, first, the node is visited and then it’s child nodes are put in a FIFO queue.

```printLevelorder(tree)
1) Create an empty queue q
2) temp_node = root /*start from root*/
3) Loop while temp_node is not NULL
a) print temp_node->data.
b) Enqueue temp_node’s children
(first left then right children) to q
c) Dequeue a node from q.```

Implementation: Here is a simple implementation of the above algorithm. The queue is implemented using an array with a maximum size of 500. We can implement a queue as a linked list also.

## C++

 `/* C++ program to print level``    ``order traversal using STL */``#include ``using` `namespace` `std;` `// A Binary Tree Node``struct` `Node {``    ``int` `data;``    ``struct` `Node *left, *right;``};` `// Iterative method to find height of Binary Tree``void` `printLevelOrder(Node* root)``{``    ``// Base Case``    ``if` `(root == NULL)``        ``return``;` `    ``// Create an empty queue for level order traversal``    ``queue q;` `    ``// Enqueue Root and initialize height``    ``q.push(root);` `    ``while` `(q.empty() == ``false``) {``        ``// Print front of queue and remove it from queue``        ``Node* node = q.front();``        ``cout << node->data << ``" "``;``        ``q.pop();` `        ``/* Enqueue left child */``        ``if` `(node->left != NULL)``            ``q.push(node->left);` `        ``/*Enqueue right child */``        ``if` `(node->right != NULL)``            ``q.push(node->right);``    ``}``}` `// Utility function to create a new tree node``Node* newNode(``int` `data)``{``    ``Node* temp = ``new` `Node;``    ``temp->data = data;``    ``temp->left = temp->right = NULL;``    ``return` `temp;``}` `// Driver program to test above functions``int` `main()``{``    ``// Let us create binary tree shown in above diagram``    ``Node* root = newNode(1);``    ``root->left = newNode(2);``    ``root->right = newNode(3);``    ``root->left->left = newNode(4);``    ``root->left->right = newNode(5);` `    ``cout << ``"Level Order traversal of binary tree is \n"``;``    ``printLevelOrder(root);``    ``return` `0;``}`

## C

 `// Iterative Queue based C program``// to do level order traversal``// of Binary Tree``#include ``#include ``#define MAX_Q_SIZE 500` `/* A binary tree node has data,``   ``pointer to left child``   ``and a pointer to right child */``struct` `node {``    ``int` `data;``    ``struct` `node* left;``    ``struct` `node* right;``};` `/* frunction prototypes */``struct` `node** createQueue(``int``*, ``int``*);``void` `enQueue(``struct` `node**, ``int``*, ``struct` `node*);``struct` `node* deQueue(``struct` `node**, ``int``*);` `/* Given a binary tree, print its nodes in level order``   ``using array for implementing queue */``void` `printLevelOrder(``struct` `node* root)``{``    ``int` `rear, front;``    ``struct` `node** queue = createQueue(&front, &rear);``    ``struct` `node* temp_node = root;` `    ``while` `(temp_node) {``        ``printf``(``"%d "``, temp_node->data);` `        ``/*Enqueue left child */``        ``if` `(temp_node->left)``            ``enQueue(queue, &rear, temp_node->left);` `        ``/*Enqueue right child */``        ``if` `(temp_node->right)``            ``enQueue(queue, &rear, temp_node->right);` `        ``/*Dequeue node and make it temp_node*/``        ``temp_node = deQueue(queue, &front);``    ``}``}` `/*UTILITY FUNCTIONS*/``struct` `node** createQueue(``int``* front, ``int``* rear)``{``    ``struct` `node** queue = (``struct` `node**)``malloc``(``        ``sizeof``(``struct` `node*) * MAX_Q_SIZE);` `    ``*front = *rear = 0;``    ``return` `queue;``}` `void` `enQueue(``struct` `node** queue, ``int``* rear,``             ``struct` `node* new_node)``{``    ``queue[*rear] = new_node;``    ``(*rear)++;``}` `struct` `node* deQueue(``struct` `node** queue, ``int``* front)``{``    ``(*front)++;``    ``return` `queue[*front - 1];``}` `/* Helper function that allocates a new node with the``   ``given data and NULL left and right pointers. */``struct` `node* newNode(``int` `data)``{``    ``struct` `node* node``        ``= (``struct` `node*)``malloc``(``sizeof``(``struct` `node));``    ``node->data = data;``    ``node->left = NULL;``    ``node->right = NULL;` `    ``return` `(node);``}` `/* Driver program to test above functions*/``int` `main()``{``    ``struct` `node* root = newNode(1);``    ``root->left = newNode(2);``    ``root->right = newNode(3);``    ``root->left->left = newNode(4);``    ``root->left->right = newNode(5);` `    ``printf``(``"Level Order traversal of binary tree is \n"``);``    ``printLevelOrder(root);` `    ``return` `0;``}`

## Java

 `// Iterative Queue based Java program``// to do level order traversal``// of Binary Tree` `/* importing the inbuilt java classes``   ``required for the program */``import` `java.util.LinkedList;``import` `java.util.Queue;` `/* Class to represent Tree node */``class` `Node {``    ``int` `data;``    ``Node left, right;` `    ``public` `Node(``int` `item)``    ``{``        ``data = item;``        ``left = ``null``;``        ``right = ``null``;``    ``}``}` `/* Class to print Level Order Traversal */``class` `BinaryTree {` `    ``Node root;` `    ``/* Given a binary tree. Print``     ``its nodes in level order``     ``using array for implementing queue  */``    ``void` `printLevelOrder()``    ``{``        ``Queue queue = ``new` `LinkedList();``        ``queue.add(root);``        ``while` `(!queue.isEmpty()) {` `            ``/* poll() removes the present head.``            ``For more information on poll() visit``            ``http://www.tutorialspoint.com/java/``            ``util/linkedlist_poll.htm */``            ``Node tempNode = queue.poll();``            ``System.out.print(tempNode.data + ``" "``);` `            ``/*Enqueue left child */``            ``if` `(tempNode.left != ``null``) {``                ``queue.add(tempNode.left);``            ``}` `            ``/*Enqueue right child */``            ``if` `(tempNode.right != ``null``) {``                ``queue.add(tempNode.right);``            ``}``        ``}``    ``}` `    ``public` `static` `void` `main(String args[])``    ``{``        ``/* creating a binary tree and entering``         ``the nodes */``        ``BinaryTree tree_level = ``new` `BinaryTree();``        ``tree_level.root = ``new` `Node(``1``);``        ``tree_level.root.left = ``new` `Node(``2``);``        ``tree_level.root.right = ``new` `Node(``3``);``        ``tree_level.root.left.left = ``new` `Node(``4``);``        ``tree_level.root.left.right = ``new` `Node(``5``);` `        ``System.out.println("Level order traversal``                            ``of binary tree is - ");``        ``tree_level.printLevelOrder();``    ``}``}`

## Python3

 `# Python program to print level``# order traversal using Queue` `# A node structure`  `class` `Node:``    ``# A utility function to create a new node``    ``def` `__init__(``self``, key):``        ``self``.data ``=` `key``        ``self``.left ``=` `None``        ``self``.right ``=` `None` `# Iterative Method to print the``# height of a binary tree`  `def` `printLevelOrder(root):``    ``# Base Case``    ``if` `root ``is` `None``:``        ``return` `    ``# Create an empty queue``    ``# for level order traversal``    ``queue ``=` `[]` `    ``# Enqueue Root and initialize height``    ``queue.append(root)` `    ``while``(``len``(queue) > ``0``):` `        ``# Print front of queue and``        ``# remove it from queue``        ``print``(queue[``0``].data)``        ``node ``=` `queue.pop(``0``)` `        ``# Enqueue left child``        ``if` `node.left ``is` `not` `None``:``            ``queue.append(node.left)` `        ``# Enqueue right child``        ``if` `node.right ``is` `not` `None``:``            ``queue.append(node.right)`  `# Driver Program to test above function``root ``=` `Node(``1``)``root.left ``=` `Node(``2``)``root.right ``=` `Node(``3``)``root.left.left ``=` `Node(``4``)``root.left.right ``=` `Node(``5``)` `print``(``"Level Order Traversal of binary tree is -"``)``printLevelOrder(root)``# This code is contributed by Nikhil Kumar Singh(nickzuck_007)`

## C#

 `// Iterative Queue based C# program``// to do level order traversal``// of Binary Tree` `using` `System;``using` `System.Collections.Generic;` `/* Class to represent Tree node */``public` `class` `Node {``    ``public` `int` `data;``    ``public` `Node left, right;` `    ``public` `Node(``int` `item)``    ``{``        ``data = item;``        ``left = ``null``;``        ``right = ``null``;``    ``}``}` `/* Class to print Level Order Traversal */``public` `class` `BinaryTree {` `    ``Node root;` `    ``/* Given a binary tree. Print``    ``its nodes in level order using``     ``array for implementing queue */``    ``void` `printLevelOrder()``    ``{``        ``Queue queue = ``new` `Queue();``        ``queue.Enqueue(root);``        ``while` `(queue.Count != 0) {` `            ``Node tempNode = queue.Dequeue();``            ``Console.Write(tempNode.data + ``" "``);` `            ``/*Enqueue left child */``            ``if` `(tempNode.left != ``null``) {``                ``queue.Enqueue(tempNode.left);``            ``}` `            ``/*Enqueue right child */``            ``if` `(tempNode.right != ``null``) {``                ``queue.Enqueue(tempNode.right);``            ``}``        ``}``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main()``    ``{``        ``/* creating a binary tree and entering``        ``the nodes */``        ``BinaryTree tree_level = ``new` `BinaryTree();``        ``tree_level.root = ``new` `Node(1);``        ``tree_level.root.left = ``new` `Node(2);``        ``tree_level.root.right = ``new` `Node(3);``        ``tree_level.root.left.left = ``new` `Node(4);``        ``tree_level.root.left.right = ``new` `Node(5);` `        ``Console.WriteLine(``"Level order traversal "``                          ``+ ``"of binary tree is - "``);``        ``tree_level.printLevelOrder();``    ``}``}` `/* This code contributed by PrinciRaj1992 */`

## Javascript

 ``

Output

```Level Order traversal of binary tree is
1 2 3 4 5 ```

Time Complexity: O(n) where n is the number of nodes in the binary tree.
Auxiliary Space: O(n) where n is the number of nodes in the binary tree.

Please write comments if you find any bugs in the above programs/algorithms or other ways to solve the same problem.

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