【leetcode】Binary Tree Right Side View（middle）

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].

思路：BFS 

class Solution {
public:
    vector<int> rightSideView(TreeNode *root) {
        vector<int> ans;
        if(root == NULL) return ans;
        queue<TreeNode *> Q;
        Q.push(root);
        
        while(!Q.empty())
        {
            ans.push_back(Q.front()->val);
            int pos = Q.size(); //当前层的元素个数
            while(pos != 0)
            {
                if(Q.front()->right != NULL) 
                    Q.push(Q.front()->right);
                if(Q.front()->left != NULL) 
                    Q.push(Q.front()->left);
                Q.pop();
                pos--;
            }
        }

        return ans;
    }
};