Unique Binary Search Trees II [Leetcode]

Given 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:  this problem can be solved recursively; if the values of the nodes in the tree we want to generate are in the range of [start, end]; then the values of the nodes in the left subtree are in the range of [start, i-1], and the values of the nodes in the right subtree are in the range of [i+1, end], where i is the value of the root and start \le i \le end.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<TreeNode *> generateTreesHelper(int start, int end) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<TreeNode *> ret;
        if(start>end){
            ret.push_back(NULL);
            return ret;
        }
        for(int i=start; i<=end; i++){
            vector<TreeNode *> leftTree = generateTreesHelper(start, i-1);
            vector<TreeNode *> rightTree = generateTreesHelper(i+1, end);
            for(int j=0; j<leftTree.size(); j++){
                for(int k=0; k<rightTree.size(); k++){
                    TreeNode* root = new TreeNode(i);
                    root->left = leftTree[j];
                    root->right = rightTree[k];
                    ret.push_back(root);
                }
            }
        }
        return ret;
    }
    vector<TreeNode *> generateTrees(int n) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        return generateTreesHelper(1, n);
    }
};