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 binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
//找到一个数作为根结点，剩余的数分别划入左子树或者右子树
    vector<TreeNode *> generateTrees(int n) {
        return create(1,n);
    }
    
    vector<TreeNode *>create(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=create(start,i-1);
            vector<TreeNode *>right=create(i+1,end);
            for(int j=0;j<left.size();j++)
            {
                for(int k=0;k<right.size();k++)
                {
                    TreeNode *r=new TreeNode(i);
                    r->left=left[j];
                    r->right=right[k];
                    res.push_back(r);
                }
            }
        }
        return res;
    }
};