不分行从上往下打印二叉树

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