Branch does not print binary tree from the top down

Downward from the print out of each node of the binary tree, the same one node print order from left to right.

Sample

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> printFromTopToBottom(TreeNode* root) {
        vector<int> res;
        if(!root) return res;
        queue<TreeNode*> q;
        q.push(root);
        while(q.size()){
            auto t = q.front();
            q.pop();
            res.push_back(t->val);
            if(t->left) q.push(t->left);
            if(t->right)q.push(t->right);
        }
        
        return res;
    }
};