/**
 * 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:
    struct node{
        TreeNode * p;
        int step;
    };
    vector<vector<int>> printFromTopToBottom(TreeNode* root) {
        vector<vector<int>> res;
        if(!root)   return res;
        queue<node> qu;
        qu.push({root, 0});
        int cur = 0;
        vector<int> tmp;
        while(!qu.empty()){
            node now = qu.front();
            qu.pop();
            if(now.step == cur) tmp.push_back({now.p->val});
            else{
                if(tmp.size() != 0) res.push_back(tmp);
                tmp.clear();
                cur = now.step;
                tmp.push_back(now.p->val);
            }
            if(now.p->left) qu.push({now.p->left, now.step + 1});
            if(now.p->right) qu.push({now.p->right, now.step + 1});
        }
        if(tmp.size() != 0) res.push_back(tmp);
        return res;
    }
};