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.

Example:

  1. Input: [1,2,3,null,5,null,4]
  2. Output: [1, 3, 4]
  3. Explanation:
  5.    1            <---
  6.  /   \
  7. 2     3         <---
  8.  \     \
  9.   5     4       <---

题意

站在一个二叉树的右边向左看，求从上到下能看到的所有元素。

思路

相当于取每一行的最右元素，层序遍历即可。

代码实现

Java

  1. class Solution {
  2.     public List<Integer> rightSideView(TreeNode root) {
  3.         List<Integer> ans = new ArrayList<>();
  4.         Queue<TreeNode> q = new LinkedList<>();
  6.         if (root != null) {
  7.             q.offer(root);
  8.         }
  10.         while (!q.isEmpty()) {
  11.             int size = q.size();
  12.             TreeNode cur = null;
  13.             while (size > 0) {
  14.                 cur = q.poll();
  15.                 if (cur.left != null) q.offer(cur.left);
  16.                 if (cur.right != null) q.offer(cur.right);
  17.                 size--;
  18.             }
  19.             ans.add(cur.val);
  20.         }
  22.         return ans;
  23.     }
  24. }