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

Runtime: 28ms

/**
 * 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*> generateTrees(int n) {
        vector<TreeNode* > result;
        if(!n) return result;
        return generateBST(1, n);
    }
    
    vector<TreeNode* > generateBST(int low, int high) {
        vector<TreeNode* > result;
        if(low > high) {
            result.push_back(NULL);
            return result;
        }
        
        for(int i = low; i <= high; i++) {
            vector<TreeNode* > leftSubtree = generateBST(low, i - 1);
            vector<TreeNode* > rightSubtree = generateBST(i + 1, high);
            
            for(int m = 0; m < leftSubtree.size(); m++) {
                for(int n = 0; n < rightSubtree.size(); n++) {
                    TreeNode* root = new TreeNode(i);
                    root->left = leftSubtree[m];
                    root->right = rightSubtree[n];
                    result.push_back(root);
                }
            }
        }
        
        return result;
    }
};