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,

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

You should return [1, 3, 4].

思路

利用层次遍历（BFS）轻松解决问题。需要注意的是，我们在这儿需要用一个空节点来分割不同的层级。

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/**
 * 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> ans;
        queue<TreeNode*> bfs;
        if(root != NULL){
            bfs.push(root);
            bfs.push(NULL);
            int tmp = 0;
            while(!bfs.empty()){
                TreeNode* cur = bfs.front();
                bfs.pop();
                if(cur != NULL){
                    tmp = cur->val;
                    if(cur->left != NULL){
                        bfs.push(cur->left);
                    }
                    if(cur->right != NULL){
                        bfs.push(cur->right);
                    }
                }
                else{
                    ans.push_back(tmp);
                    tmp = 0;
                    if(!bfs.empty()) bfs.push(NULL);
                }
            }
        }
        return ans;
    }
};