Leetcode108: 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

/** * 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*> createTree(int start, int end)    {        vector<TreeNode*> res;        if(start>end)        {            res.push_back(NULL);            return res;        }        for(int i = start; i <= end; i++)        {            vector<TreeNode*> left = createTree(start, i-1);            vector<TreeNode*> right = createTree(i+1, end);            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];                    res.push_back(root);                }            }        }        return res;    }    vector<TreeNode*> generateTrees(int n) {       return createTree(1, n);    }};