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Iterative Method To Print Left View of a Binary Tree
• Difficulty Level : Medium
• Last Updated : 31 Aug, 2020

Given a Binary Tree, print it’s left view. Left view of a Binary Tree is a set of nodes visible when tree is seen from the left side . Examples:

```Input :        1
/   \
2     3
/ \   / \
4   5 6   7
Output : 1 2 4

Input :         1
/   \
2     3
\   /
4 5
\
6
/ \
7   8
Output : 1 2 4 6 7
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

We have already discussed this problem using the Recursion method, here iterative approach is used to solve the above problem.

The idea is to do level order traversal of the Tree using a queue and print the first node at each level.

While doing level order traversal, after traversing all node at each level, push a NULL delimiter to mark the end of the current level. So, do the level order traversal of the tree. Print the first node at each level in the tree and push the children of all nodes at each level in the queue until a NULL delimiter is encountered.

Below is the implementation of above approach:

## C++

 `// C++ program to print the``// left view of Binary Tree`` ` `#include `` ` `using` `namespace` `std;`` ` `// A Binary Tree Node``struct` `node {``    ``int` `data;``    ``struct` `node *left, *right;``};`` ` `// A utility function to create a new``// Binary Tree node``struct` `node* newNode(``int` `item)``{``    ``struct` `node* temp = ``new` `node;``    ``temp->data = item;``    ``temp->left = NULL;``    ``temp->right = NULL;``    ``return` `temp;``}`` ` `// Utility function to print the left view of``// the binary tree``void` `leftViewUtil(``struct` `node* root, queue& q)``{``    ``if` `(root == NULL)``        ``return``;`` ` `    ``// Push root``    ``q.push(root);`` ` `    ``// Delimiter``    ``q.push(NULL);`` ` `    ``while` `(!q.empty()) {``        ``node* temp = q.front();`` ` `        ``if` `(temp) {`` ` `            ``// Prints first node``            ``// of each level``            ``cout << temp->data << ``" "``;`` ` `            ``// Push children of all nodes at``            ``// current level``            ``while` `(q.front() != NULL) {`` ` `                ``// If left child is present``                ``// push into queue``                ``if` `(temp->left)``                    ``q.push(temp->left);`` ` `                ``// If right child is present``                ``// push into queue``                ``if` `(temp->right)``                    ``q.push(temp->right);`` ` `                ``// Pop the current node``                ``q.pop();`` ` `                ``temp = q.front();``            ``}`` ` `            ``// Push delimiter``            ``// for the next level``            ``q.push(NULL);``        ``}`` ` `        ``// Pop the delimiter of``        ``// the previous level``        ``q.pop();``    ``}``}`` ` `// Function to print the leftView``// of Binary Tree``void` `leftView(``struct` `node* root)``{``    ``// Queue to store all``    ``// the nodes of the tree``    ``queue q;`` ` `    ``leftViewUtil(root, q);``}`` ` `// Driver Code``int` `main()``{``    ``struct` `node* root = newNode(10);``    ``root->left = newNode(12);``    ``root->right = newNode(3);``    ``root->left->right = newNode(4);``    ``root->right->left = newNode(5);``    ``root->right->left->right = newNode(6);``    ``root->right->left->right->left = newNode(18);``    ``root->right->left->right->right = newNode(7);`` ` `    ``leftView(root);`` ` `    ``return` `0;``}`

## Java

 `// Java program to print the``// left view of Binary Tree``import` `java.util.*;`` ` `class` `GFG``{`` ` `// A Binary Tree Node``static` `class` `node``{``    ``int` `data;``    ``node left, right;``};`` ` `// A utility function to create a new``// Binary Tree node``static` `node newNode(``int` `item)``{``    ``node temp = ``new` `node();``    ``temp.data = item;``    ``temp.left = ``null``;``    ``temp.right = ``null``;``    ``return` `temp;``}``static` `Queue q;`` ` `// Utility function to print the left view of``// the binary tree``static` `void` `leftViewUtil( node root )``{``    ``if` `(root == ``null``)``        ``return``;`` ` `    ``// add root``    ``q.add(root);`` ` `    ``// Delimiter``    ``q.add(``null``);`` ` `    ``while` `(q.size() > ``0``) ``    ``{``        ``node temp = q.peek();`` ` `        ``if` `(temp != ``null``)``        ``{`` ` `            ``// Prints first node``            ``// of each level``            ``System.out.print(temp.data + ``" "``);`` ` `            ``// add children of all nodes at``            ``// current level``            ``while` `(q.peek() != ``null``)``            ``{`` ` `                ``// If left child is present``                ``// add into queue``                ``if` `(temp.left != ``null``)``                    ``q.add(temp.left);`` ` `                ``// If right child is present``                ``// add into queue``                ``if` `(temp.right != ``null``)``                    ``q.add(temp.right);`` ` `                ``// remove the current node``                ``q.remove();`` ` `                ``temp = q.peek();``            ``}`` ` `            ``// add delimiter``            ``// for the next level``            ``q.add(``null``);``        ``}`` ` `        ``// remove the delimiter of``        ``// the previous level``        ``q.remove();``    ``}``}`` ` `// Function to print the leftView``// of Binary Tree``static` `void` `leftView( node root)``{``    ``// Queue to store all``    ``// the nodes of the tree``    ``q = ``new` `LinkedList();`` ` `    ``leftViewUtil(root);``}`` ` `// Driver Code``public` `static` `void` `main(String args[])``{``    ``node root = newNode(``10``);``    ``root.left = newNode(``12``);``    ``root.right = newNode(``3``);``    ``root.left.right = newNode(``4``);``    ``root.right.left = newNode(``5``);``    ``root.right.left.right = newNode(``6``);``    ``root.right.left.right.left = newNode(``18``);``    ``root.right.left.right.right = newNode(``7``);`` ` `    ``leftView(root);``}``}`` ` `// This code is contributed by Arnab Kundu`

## Python3

 `# Python3 program to print the ``# left view of Binary Tree`` ` `# Binary Tree Node ``""" utility that allocates a newNode ``with the given key """``class` `newNode: `` ` `    ``# Construct to create a newNode ``    ``def` `__init__(``self``, key): ``        ``self``.data ``=` `key``        ``self``.left ``=` `None``        ``self``.right ``=` `None``        ``self``.hd``=``0`` ` `# Utility function to print the left ``# view of the binary tree ``def` `leftViewUtil(root, q) :`` ` `    ``if` `(root ``=``=` `None``) :``        ``return`` ` `    ``# append root ``    ``q.append(root) `` ` `    ``# Delimiter ``    ``q.append(``None``) `` ` `    ``while` `(``len``(q)): ``        ``temp ``=` `q[``0``]`` ` `        ``if` `(temp):`` ` `            ``# Prints first node of each level ``            ``print``(temp.data, end ``=` `" "``)`` ` `            ``# append children of all nodes ``            ``# at current level ``            ``while` `(q[``0``] !``=` `None``) :``                ``temp ``=` `q[``0``]``                 ` `                ``# If left child is present ``                ``# append into queue ``                ``if` `(temp.left) :``                    ``q.append(temp.left) `` ` `                ``# If right child is present ``                ``# append into queue ``                ``if` `(temp.right) :``                    ``q.append(temp.right) `` ` `                ``# Pop the current node ``                ``q.pop(``0``) ``             ` `            ``# append delimiter ``            ``# for the next level ``            ``q.append(``None``) ``         ` `        ``# Pop the delimiter of ``        ``# the previous level ``        ``q.pop(``0``) ``     ` `# Function to print the leftView ``# of Binary Tree ``def` `leftView(root):`` ` `    ``# Queue to store all ``    ``# the nodes of the tree ``    ``q ``=` `[] `` ` `    ``leftViewUtil(root, q) `` ` `# Driver Code ``if` `__name__ ``=``=` `'__main__'``:`` ` `    ``root ``=` `newNode(``10``) ``    ``root.left ``=` `newNode(``12``) ``    ``root.right ``=` `newNode(``3``) ``    ``root.left.right ``=` `newNode(``4``) ``    ``root.right.left ``=` `newNode(``5``) ``    ``root.right.left.right ``=` `newNode(``6``) ``    ``root.right.left.right.left ``=` `newNode(``18``) ``    ``root.right.left.right.right ``=` `newNode(``7``)``    ``leftView(root)`` ` `# This code is contributed by``# Shubham Singh(SHUBHAMSINGH10)`

## C#

 `// C# program to print the``// left view of Binary Tree``using` `System;``using` `System.Collections.Generic; ``     ` `class` `GFG``{`` ` `// A Binary Tree Node``public` `class` `node``{``    ``public` `int` `data;``    ``public` `node left, right;``};`` ` `// A utility function to create a new``// Binary Tree node``static` `node newNode(``int` `item)``{``    ``node temp = ``new` `node();``    ``temp.data = item;``    ``temp.left = ``null``;``    ``temp.right = ``null``;``    ``return` `temp;``}``static` `Queue q = ``new` `Queue();`` ` `// Utility function to print the left view of``// the binary tree``static` `void` `leftViewUtil( node root )``{``    ``if` `(root == ``null``)``        ``return``;`` ` `    ``// add root``    ``q.Enqueue(root);`` ` `    ``// Delimiter``    ``q.Enqueue(``null``);`` ` `    ``while` `(q.Count > 0) ``    ``{``        ``node temp = q.Peek();`` ` `        ``if` `(temp != ``null``)``        ``{`` ` `            ``// Prints first node``            ``// of each level``            ``Console.Write(temp.data + ``" "``);`` ` `            ``// add children of all nodes at``            ``// current level``            ``while` `(q.Peek() != ``null``)``            ``{`` ` `                ``// If left child is present``                ``// add into queue``                ``if` `(temp.left != ``null``)``                    ``q.Enqueue(temp.left);`` ` `                ``// If right child is present``                ``// add into queue``                ``if` `(temp.right != ``null``)``                    ``q.Enqueue(temp.right);`` ` `                ``// remove the current node``                ``q.Dequeue();`` ` `                ``temp = q.Peek();``            ``}`` ` `            ``// add delimiter``            ``// for the next level``            ``q.Enqueue(``null``);``        ``}`` ` `        ``// remove the delimiter of``        ``// the previous level``        ``q.Dequeue();``    ``}``}`` ` `// Function to print the leftView``// of Binary Tree``static` `void` `leftView( node root)``{``    ``// Queue to store all``    ``// the nodes of the tree``    ``q = ``new` `Queue();`` ` `    ``leftViewUtil(root);``}`` ` `// Driver Code``public` `static` `void` `Main(String []args)``{``    ``node root = newNode(10);``    ``root.left = newNode(12);``    ``root.right = newNode(3);``    ``root.left.right = newNode(4);``    ``root.right.left = newNode(5);``    ``root.right.left.right = newNode(6);``    ``root.right.left.right.left = newNode(18);``    ``root.right.left.right.right = newNode(7);`` ` `    ``leftView(root);``}``}`` ` `// This code is contributed by 29AjayKumar`

Output:

```10 12 4 6 18
```

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