leetcode95 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*> generateTrees(int n) {
        vector<TreeNode*> ans;
        ans=generate(1,n);
        return ans;
    }
    vector<TreeNode*> generate(int a,int b)
    {
        vector<TreeNode*> ans;
        if(a>b)
            return ans;
        if(a==b)
        {
            TreeNode *temp=new TreeNode(a);
            ans.push_back(temp);
            return ans;
        }
        vector<TreeNode*> vt1=generate(a+1,b);
        for(int i=0;i<vt1.size();i++)
        {
            TreeNode *tr1=new TreeNode(a);//不能放到外边，否则，ans中的tr1将会变成全部相同一个。（tr1相同，tr1->right不同，故应多分tr1）
            tr1->right=vt1[i];
            ans.push_back(tr1);
        }
        
        vector<TreeNode*> vt2=generate(a,b-1);
        for(int i=0;i<vt2.size();i++)
        {
            TreeNode *tr2=new TreeNode(b);
            tr2->left=vt2[i];
            ans.push_back(tr2);
        }
        
        for(int i=a+1;i<=b-1;i++)
        {
            vector<TreeNode*> vt31=generate(a,i-1);
            vector<TreeNode*> vt32=generate(i+1,b);
            for(int j=0;j<vt31.size();j++)
            {
                for(int k=0;k<vt32.size();k++)
                {
                    TreeNode *tr3=new TreeNode(i);
                    tr3->left=vt31[j];
                    tr3->right=vt32[k];
                    ans.push_back(tr3);
                }
            }
            
            
        }
        return ans;
    }
};