199.二叉树的右视图

题目描述

原题

给定一棵二叉树,想象自己站在它的右侧,按照从顶部到底部的顺序,返回从右侧所能看到的节点值。

示例:

输入: [1,2,3,null,5,null,4]
输出: [1, 3, 4]
解释:
1 <---
/ \
2 3 <---
\ \
5 4 <---

题解

/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public List<Integer> rightSideView(TreeNode root) {
List<Integer> list = new ArrayList<>();
if(root == null) return list;
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
//层序遍历 只保存最后一个数即可
while(!queue.isEmpty()){
int size = queue.size();
for(int i = 0; i < size;i++){
TreeNode node = queue.poll();
if(i == size -1)
list.add(node.val);
if(node.left!=null)
queue.offer(node.left);
if(node.right!=null)
queue.offer(node.right);
}
}
return list;
}
}