输入如下图所示二叉树[8, 12, 2, null, null, 6, null, 4, null, null, null]
    8
   /   12  2
     /
    6
   /
  4

输出：[8, 12, 2, 6, 4]

/**
 * 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;
    }
};