95 Unique Binary Search Trees II

Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1...n.
For example,
Given n = 3, your program should return all 5 unique BST's shown below.

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3

Solution:

/**
 * 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<TreeNode*> helper(vector<int>& nums, int i, int j) {
        vector<TreeNode*> res;
        if ( i > j ) { res.push_back(NULL); return res; }
        for ( int k = i; k <= j; k++ ) {
            auto res_left = helper(nums, i, k-1);
            auto res_right = helper(nums, k+1, j);
            int nl = res_left.size(), nr = res_right.size();
            for ( int il = 0; il <= nl-1; il++ ) {
                for ( int ir = 0; ir <= nr-1; ir++ ) {
                    TreeNode *root = new TreeNode(nums[k]);
                    root->left = res_left[il];
                    root->right = res_right[ir];
                    res.push_back(root);
                }
            }
        }
        return res;
    }
    vector<TreeNode*> generateTrees(int n) {
        vector<TreeNode*> res;
        if ( n == 0 ) return res;
        vector<int> nums;
        for ( int i = 1; i <= n; i++ ) nums.push_back(i);
        res = helper(nums, 0, n-1);
        return res;
    }
};