0199. Binary Tree Right Side View (M)

Binary Tree Right Side View (M)

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:

Input: [1,2,3,null,5,null,4]
Output: [1, 3, 4]
Explanation:

   1            <---
 /   
2     3         <---
      
  5     4       <---

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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。

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代码实现

Java

class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        Queue<TreeNode> q = new LinkedList<>();

        if (root != null) {
            q.offer(root);
        }

        while (!q.isEmpty()) {
            int size = q.size();
            TreeNode cur = null;
            while (size > 0) {
                cur = q.poll();
                if (cur.left != null) q.offer(cur.left);
                if (cur.right != null) q.offer(cur.right);
                size--;
            }
            ans.add(cur.val);
        }

        return ans;
    }
}