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].

這道題就是BT的Level Order Traversal，每次要換一層的時候，記錄當前節點

 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 }