102.cpp

// preOrderReversion-3ms.cpp
/**
 * 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 {
    void preOrder(int i, TreeNode* root, vector<vector<int>>& nums) {
        if (root == nullptr)
            return;
        if (nums.size() == i)
            nums.push_back({});
        nums[i].push_back(root->val);
        preOrder(i + 1, root->left, nums);
        preOrder(i + 1, root->right, nums);
    }

public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> result;
        preOrder(0, root, result);
        return result;
    }
};