[LeetCode] 199. Binary Tree Right Side View Java

题目：

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].

题意及分析：给出一棵二叉树，求每一层最右节点组成的序列。宽度遍历，取出每一层的最后一个点即可。

代码：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if(root==null) return res;
        Queue<TreeNode> queue = new LinkedList<>();
        Queue<TreeNode> tempQueue = new LinkedList<>();
        queue.add(root);
        while(!queue.isEmpty()||!tempQueue.isEmpty()){
            if(!queue.isEmpty()){
                TreeNode node = queue.poll();
                if(node.left!=null){
                    tempQueue.add(node.left);
                }
                if(node.right!=null){
                    tempQueue.add(node.right);
                }
                if(queue.isEmpty()) //最后一个点
                    res.add(node.val);
            }else{
                queue = new LinkedList<>(tempQueue);
                tempQueue.clear();
            }
        }
        return res;
    }
}