Leetcode: 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            <---

 /   \

2     3         <---

 \     \

  5     4       <---

You should return [1, 3, 4].

这道题就是BT的Level Order Traversal，每次要换一层的时候，记录当前节点

 1 /**

 2  * Definition for binary tree

 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         ArrayList<Integer> res = new ArrayList<Integer>();

13         if (root == null) return res;

14         LinkedList<TreeNode> queue = new LinkedList<TreeNode>();

15         queue.offer(root);

16         int PNum = 1;

17         int CNum = 0;

18         while (!queue.isEmpty()) {

19             TreeNode cur = queue.poll();

20             PNum--;

21             if (cur.left != null) {

22                 queue.offer(cur.left);

23                 CNum++;

24             }

25             if (cur.right != null) {

26                 queue.offer(cur.right);

27                 CNum++;

28             }

29             if (PNum == 0) {

30                 res.add(cur.val);

31                 PNum = CNum;

32                 CNum = 0;

33             }

34         }

35         return res;

36     }

37 }