199 Binary Tree Right Side View

来源：Leetcode

原帖：https://leetcode.com/problems/binary-tree-right-side-view/

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.
For example:
Given the following binary tree,
       1            <---
     /   \
    2     3         <---
     \     \
      5     4       <---
You should return [1, 3, 4].

代码：

 /**  
  * Definition for a binary tree node.  
  * struct TreeNode {  
  *   int val;  
  *   TreeNode *left;  
  *   TreeNode *right;  
  *   TreeNode(int x) : val(x), left(NULL), right(NULL) {}  
  * };  
  */  
 class Solution {  
 public:  
   void dfs(TreeNode* root, int level, vector<int>& res) {  
     if (!root) return;  
     if (res.size() == level) {  
       res.push_back(root->val);  
     } else {  
       res[level] = root->val;  
     }  
     dfs(root->left, level+1, res);  
     dfs(root->right, level+1, res);  
   }  
     
   vector<int> rightSideView(TreeNode* root) {  
     if (!root) return {};  
     vector<int> res;  
     dfs(root, 0, res);  
     return res;  
   }  
 };