Print Left View of a Binary Tree

Given a Binary Tree, print left view of it. Left view of a Binary Tree is set of nodes visible when tree is visited from left side. Left view of following tree is 12, 10, 25.

          12
       /     \
     10       30
            /    \
          25      40 

The left view contains all nodes that are first nodes in their levels. A simple solution is to do level order traversal and print the first node in every level.

The problem can also be solved using simple recursive traversal. We can keep track of level of a node by passing a parameter to all recursive calls. The idea is to keep track of maximum level also. Whenever we see a node whose level is more than maximum level so far, we print the node because this is the first node in its level (Note that we traverse the left subtree before right subtree).

Solution:

// Java program to print left view 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 to print the left view */ class BinaryTree {     Node root;     static int  max_level = 0;       // recursive function to print left view     void leftViewUtil(Node node, int level)     {         // Base Case         if (node==null) return;           // If this is the first node of its level         if (max_level < level)         {             System.out.print(" " + node.data);             max_level = level;         }           // Recur for left and right subtrees         leftViewUtil(node.left, level+1);         leftViewUtil(node.right, level+1);     }       // A wrapper over leftViewUtil()     void leftView()     {         leftViewUtil(root, 1);     }       /* testing for example nodes */     public static void main(String args[])     {         /* creating a binary tree and entering the nodes */         BinaryTree tree = new BinaryTree();         tree.root = new Node(12);         tree.root.left = new Node(10);         tree.root.right = new Node(30);         tree.root.right.left = new Node(25);         tree.root.right.right = new Node(40);           tree.leftView();     } }