leftViewBT.java

import java.util.HashMap;

  //to store a Binary Tree node
  class Node
  {
      int data;
      Node left = null, right = null;

      Node(int data) {
          this.data = data;
      }
  }
  public class InterviewBit
  {
      // traverse nodes in pre-order way
      public static void leftViewUtil(Node root, int level, HashMap<Integer, Integer> map)
      {
          if (root == null) {
              return;
          }

          // if you are visiting the level for the first time
          // insert the current node and level info to the map
          if (!map.containsKey(level)) {
              map.put(level, root.data);
          }

          leftViewUtil(root.left, level + 1, map);
          leftViewUtil(root.right, level + 1, map);
      }

      // to print left view of binary tree
      public static void leftView(Node root)
      {
          // create an empty HashMap to store first node of each level
          HashMap<Integer, Integer> map = new HashMap<>();

          // traverse the tree and find out the first nodes of each level
          leftViewUtil(root, 1, map);

          // iterate through the HashMap and print the left view
          for (int i = 0; i <map.size(); i++) {
              System.out.print(map.get(i) + " ");
          }
      }

      public static void main(String[] args)
      {
          Node root = new Node(4);
          root.left = new Node(2);
          root.right = new Node(6);
          root.left.left = new Node(1);
          root.left.left = new Node(3);
          root.right.left = new Node(5);
          root.right.right = new Node(7);
          root.right.left.left = new Node(9);

          leftView(root);
      }
  }