输入如下图所示二叉树[, , , null, null, , null, , null, null, null]

   / \

     /

   /

输出：[, , , , ]

/**

 * 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> res;

    queue<TreeNode* > que;

    void bfs()

    {

        while(!que.empty()){

            TreeNode* p = que.front();

            que.pop();

            res.push_back(p->val);

            if(p->left != NULL)

                que.push(p->left);

            if(p->right != NULL)

                que.push(p->right);

        }

    }

    vector<int> printFromTopToBottom(TreeNode* root) {

        if(root == NULL) return res;

        que.push(root);

        bfs();

        return res;

    }

};