LeetCode199 Binary Tree Right Side View

LeetCode199 Binary Tree Right Side View

问题来源LeetCode199

问题描述

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].

这道题就是假设你站在二叉树的右边，返回能够看到的节点的值。

问题分析

这道题其实就是对树进行层遍历，然后保存每层最后一个元素的值。层遍历实现的方法很多，这里我用的是队列。最后出列的元素就是该层的最后一个元素。

代码如下

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public List<Integer> rightSideView(TreeNode root) {
    List<Integer> res = new ArrayList<>();
    if(root==null) return res;
    Queue<TreeNode> queue = new ArrayDeque<>();
    Queue<TreeNode> queue2 = new ArrayDeque<>();
    queue.add(root);
    while (!queue.isEmpty()){
        TreeNode node  =queue.poll();
        if(node.left!=null) queue2.add(node.left);
        if(node.right!=null) queue2.add(node.right);
        if(queue.isEmpty()){
            res.add(node.val);
            queue=queue2;
            queue2=new ArrayDeque<>();
        }
    }
    return res;
}

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