[199]二叉树的右视图

//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       <---
// 
// Related Topics 树 深度优先搜索 广度优先搜索 递归 队列 
// 👍 412 👎 0

/*
* 199 二叉树的右视图
* 2021-03-01 14:17:44
* @author oxygenbytes
*/ 
#include "leetcode.h" 
//leetcode submit region begin(Prohibit modification and deletion)
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> rightSideView(TreeNode* root) {
        if(!root) return {};
        queue<TreeNode*> q;
        q.push(root);
        vector<int> ans;
        while(q.size()){
            ans.push_back(q.back()->val);
            int n = q.size();
            for(int i = n - 1;i >= 0;i--){
                auto ptr = q.front();q.pop();
                if(ptr->left) q.push(ptr->left);
                if(ptr->right) q.push(ptr->right);
            }
        }
        return ans;
    }
};
//leetcode submit region end(Prohibit modification and deletion)