【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;

    }

};