/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void insert(TreeNode* node, int level,unordered_map<int, vector<int>>& order){
        if(node==nullptr)
            return;
        if(order.find(level)==order.end())
            order[level] ={node->val};
        else
            order[level].push_back(node->val);

        insert(node->left, level+1, order);
        insert(node->right, level+1, order);
    }
    vector<vector<int>> levelOrder(TreeNode* root) {
        if(root==nullptr)
            return {};
        unordered_map<int, vector<int>> order;
        insert(root, 0, order);
        vector<vector<int>> ans;
        for(int i = 0;i<order.size();i++){
            ans.push_back(order[i]);
        }
        return ans;
    }
};