LeetCode 199.Binary Tree Right Side View

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].

題目要求：自上而下輸出從右邊往左邊觀察到的二叉樹的結點。

思路：從右邊開始添加，如果當前結點的高度height大於當前的最大高度，則將該結點加入vector。需要注意的是，由於每個高度只取最前面的結點，因此當前的最大高度爲vector中元素的個數。然後再先後遞歸右子樹和左子樹。

代碼如下：

/**
 * 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:
    vector<int> rightSideView(TreeNode* root) {
        vector<int>result;
        f(root,result,0);
        return result;
    }
    void f(TreeNode* node,vector<int> &result, int height){
        if(node == NULL)
            return ;
        else
            height++;
            
        if(height > result.size())
            result.push_back(node->val);
            
        f(node->right,result,height);
        f(node->left,result,height);
    }
};