Unique Binary Search Trees II

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

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

class Solution {public:    vector<TreeNode *> generateTrees(int n) {            vector<TreeNode *> result;            result = generateTreesUtil(1,n);            return result;    }    public:    vector<TreeNode *> generateTreesUtil(int start, int end)    {        vector<TreeNode *> result;        if(start == end)        {            result.push_back(new TreeNode(start));            return result;        }        else if(start > end)        {            result.push_back(NULL);            return result;        }                vector<TreeNode *>left;        vector<TreeNode *>right;                for(int i=start; i<=end; i++)        {            left = generateTreesUtil(start, i-1);            right = generateTreesUtil(i+1, end);                        // 得到left和right的combination            for(int j=0; j<left.size(); j++)                for(int k=0; k<right.size(); k++)                {                    TreeNode *root = new TreeNode(i);                    root->left = left[j];                    root->right = right[k];                                        result.push_back(root);                }        }                return result;    }};