Given an array of elements, the task is to insert these elements in level order and construct a tree.

Input : arr[] = {10, 20, 30, 40, 50, 60} Output : 10 / \ 20 30 / \ / 40 50 60

The task is to construct a whole tree from a given array. To insert in level order in an already constructed tree, please see Insertion in a Binary Tree in level order

The task is to store data in a binary tree but in level order.

To do so, we will proceed as follows:

1. Whenever a new Node is added to the binary tree, the address of the node is pushed into a queue.

2. A Node address will stay in the queue until both its children’s Nodes do not get filled.

3. Once both the children’s Nodes get filled up, the parent Node is popped from the queue.

Here is the code to perform the above mentioned data entry.

## C++

`// CPP program to construct a binary tree in level order.` `#include <bits/stdc++.h>` `using` `namespace` `std;` `struct` `Node {` ` ` `int` `key;` ` ` `Node* left;` ` ` `Node* right;` `};` `// Function to create a node with 'value' as the data` `// stored in it.` `// Both the children of this new Node are initially null.` `struct` `Node* newNode(` `int` `value)` `{` ` ` `Node* n = ` `new` `Node;` ` ` `n->key = value;` ` ` `n->left = NULL;` ` ` `n->right = NULL;` ` ` `return` `n;` `}` `struct` `Node* insertValue(` `struct` `Node* root, ` `int` `value,` ` ` `queue<Node *>& q)` `{` ` ` `Node* node = newNode(value);` ` ` `if` `(root == NULL)` ` ` `root = node;` ` ` `// The left child of the current Node is` ` ` `// used if it is available.` ` ` `else` `if` `(q.front()->left == NULL)` ` ` `q.front()->left = node;` ` ` `// The right child of the current Node is used` ` ` `// if it is available. Since the left child of this` ` ` `// node has already been used, the Node is popped` ` ` `// from the queue after using its right child.` ` ` `else` `{` ` ` `q.front()->right = node;` ` ` `q.pop();` ` ` `}` ` ` `// Whenever a new Node is added to the tree, its` ` ` `// address is pushed into the queue.` ` ` `// So that its children Nodes can be used later.` ` ` `q.push(node);` ` ` `return` `root;` `}` `// This function mainly calls insertValue() for` `// all array elements. All calls use same queue.` `Node* createTree(` `int` `arr[], ` `int` `n)` `{` ` ` `Node* root = NULL;` ` ` `queue<Node*> q;` ` ` `for` `(` `int` `i = 0; i < n; i++)` ` ` `root = insertValue(root, arr[i], q);` ` ` `return` `root;` `}` `// This is used to verify the logic.` `void` `levelOrder(` `struct` `Node* root)` `{` ` ` `if` `(root == NULL)` ` ` `return` `;` ` ` `queue<Node*> n;` ` ` `n.push(root);` ` ` `while` `(!n.empty()) {` ` ` `cout << n.front()->key << ` `" "` `;` ` ` `if` `(n.front()->left != NULL)` ` ` `n.push(n.front()->left);` ` ` `if` `(n.front()->right != NULL)` ` ` `n.push(n.front()->right);` ` ` `n.pop();` ` ` `}` `}` `// Driver code` `int` `main()` `{` ` ` `int` `arr[] = { 10, 20, 30, 40, 50, 60 };` ` ` `int` `n = ` `sizeof` `(arr) / ` `sizeof` `(arr[0]);` ` ` `Node* root = createTree(arr, n);` ` ` `levelOrder(root);` ` ` `return` `0;` `}` |

## Java

`// JAVA program to cona` `// binary tree in level` `// order.` `import` `java.util.*;` `class` `GFG{` `static` `class` `Node` `{` ` ` `int` `key;` ` ` `Node left;` ` ` `Node right;` `};` ` ` `static` `Node root = ` `null` `;` `static` `Queue<Node> q =` ` ` `new` `LinkedList<>();` ` ` `// Function to create a node` `// with 'value' as the data` `// stored in it.` `// Both the children of this` `// new Node are initially null.` `static` `Node newNode(` `int` `value)` `{` ` ` `Node n = ` `new` `Node();` ` ` `n.key = value;` ` ` `n.left = ` `null` `;` ` ` `n.right = ` `null` `;` ` ` `return` `n;` `}` `static` `void` `insertValue(` `int` `value)` `{` ` ` `Node node = newNode(value);` ` ` `if` `(root == ` `null` `)` ` ` `root = node;` ` ` `// The left child of the` ` ` `// current Node is used` ` ` `// if it is available.` ` ` `else` `if` `(q.peek().left == ` `null` `)` ` ` `q.peek().left = node;` ` ` `// The right child of the current` ` ` `// Node is used if it is available.` ` ` `// Since the left child of this` ` ` `// node has already been used, the` ` ` `// Node is popped from the queue` ` ` `// after using its right child.` ` ` `else` ` ` `{` ` ` `q.peek().right = node;` ` ` `q.remove();` ` ` `}` ` ` `// Whenever a new Node is added` ` ` `// to the tree, its address is` ` ` `// pushed into the queue. So that` ` ` `// its children Nodes can be used` ` ` `// later.` ` ` `q.add(node);` `}` `// This function mainly calls` `// insertValue() for all array` `// elements. All calls use same` `// queue.` `static` `void` `createTree(` `int` `arr[],` ` ` `int` `n)` `{` ` ` `for` `(` `int` `i = ` `0` `; i < n; i++)` ` ` `insertValue(arr[i]);` `}` `// This is used to verify` `// the logic.` `static` `void` `levelOrder(Node root)` `{` ` ` `if` `(root == ` `null` `)` ` ` `return` `;` ` ` `Queue<Node> n =` ` ` `new` `LinkedList<>();` ` ` `n.add(root);` ` ` `while` `(!n.isEmpty())` ` ` `{` ` ` `System.out.print(n.peek().key +` ` ` `" "` `);` ` ` `if` `(n.peek().left != ` `null` `)` ` ` `n.add(n.peek().left);` ` ` `if` `(n.peek().right != ` `null` `)` ` ` `n.add(n.peek().right);` ` ` `n.remove();` ` ` `}` `}` `// Driver code` `public` `static` `void` `main(String[] args)` `{` ` ` `int` `arr[] = {` `10` `, ` `20` `, ` `30` `,` ` ` `40` `, ` `50` `, ` `60` `};` ` ` `int` `n = arr.length;` ` ` `createTree(arr, n);` ` ` `levelOrder(root);` `}` `}` `// This code is contributed by Rajput-Ji` |

## Python 3

`# Python3 program to construct` `# a binary tree in level order.` `# Importing Queue for use in` `# Level Order Traversal` `import` `queue` `# Node class for holding the Binary Tree` `class` `node:` ` ` `def` `__init__(` `self` `, data ` `=` `None` `):` ` ` `self` `.data ` `=` `data` ` ` `self` `.left ` `=` `None` ` ` `self` `.right ` `=` `None` `Q ` `=` `queue.Queue()` `# Helper function helps us in adding data` `# to the tree in Level Order` `def` `insertValue(data, root):` ` ` `newnode ` `=` `node(data)` ` ` `if` `Q.empty() !` `=` `True` `:` ` ` `temp ` `=` `Q.get()` ` ` `if` `root ` `=` `=` `None` `:` ` ` `root ` `=` `newnode` ` ` ` ` `# The left child of the current Node is` ` ` `# used if it is available.` ` ` `elif` `temp.left ` `=` `=` `None` `:` ` ` `temp.left ` `=` `newnode` ` ` ` ` `# The right child of the current Node is used` ` ` `# if it is available. Since the left child of this` ` ` `# node has already been used, the Node is popped` ` ` `# from the queue after using its right child.` ` ` `elif` `temp.right ` `=` `=` `None` `:` ` ` `temp.right ` `=` `newnode` ` ` `atemp ` `=` `Q.get()` ` ` ` ` `# Whenever a new Node is added to the tree,` ` ` `# its address is pushed into the queue.` ` ` `# So that its children Nodes can be used later.` ` ` `Q.put(newnode)` ` ` `return` `root` `# Function which calls add which is responsible` `# for adding elements one by one` `def` `createTree(a, root):` ` ` `for` `i ` `in` `range` `(` `len` `(a)):` ` ` `root ` `=` `insertValue(a[i], root)` ` ` `return` `root` `# Function for printing level order traversal` `def` `levelOrder(root):` ` ` `Q ` `=` `queue.Queue()` ` ` `Q.put(root)` ` ` `while` `Q.empty() !` `=` `True` `:` ` ` `temp ` `=` `Q.get()` ` ` `print` `(temp.data, end ` `=` `' '` `)` ` ` `if` `temp.left !` `=` `None` `:` ` ` `Q.put(temp.left)` ` ` `if` `temp.right !` `=` `None` `:` ` ` `Q.put(temp.right)` `# Driver Code` `a ` `=` `[ ` `10` `, ` `20` `, ` `30` `, ` `40` `, ` `50` `, ` `60` `]` `root ` `=` `None` `root ` `=` `createTree(a, root)` `levelOrder(root)` `# This code is contributed by code_freak` |

**Output:**

10 20 30 40 50 60

Time Complexity: O(n)

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the **DSA Self Paced Course** at a student-friendly price and become industry ready.