Binary Tree Right Side View

Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

For example:

Given the following binary tree,

  1.    1            <---
  2.  /   \
  3. 2     3         <---
  4.  \     \
  5.   5     4       <---

You should return [1, 3, 4].

Solution:

  1. /**
  2.  * Definition for a binary tree node.
  3.  * public class TreeNode {
  4.  *     int val;
  5.  *     TreeNode left;
  6.  *     TreeNode right;
  7.  *     TreeNode(int x) { val = x; }
  8.  * }
  9.  */
  10. public class Solution {
  11.   public List<Integer> rightSideView(TreeNode root) {
  12.     List<Integer> res = new ArrayList<Integer>();
  14.     if (root == null)
  15.       return res;
  17.     Queue<TreeNode> queue = new LinkedList<TreeNode>();
  18.     queue.add(root);
  19.     queue.add(null);
  21.     List<Integer> list = new ArrayList<Integer>();
  23.     while (!queue.isEmpty()) {
  24.       TreeNode node = queue.poll();
  26.       if (node != null) {
  27.         list.add(node.val);
  29.         if (node.left != null)
  30.           queue.add(node.left);
  32.         if (node.right != null)
  33.           queue.add(node.right);
  34.       } else {
  35.         res.add(list.get(list.size() - 1));
  37.         if (!queue.isEmpty())
  38.           queue.add(null);
  39.       }
  40.     }
  42.     return res;
  43.   }
  44. }