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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:

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

题解

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/**
 * 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;
    }
}