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

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<TreeNode*> generateTrees(int n) {
13         vector<TreeNode*> ans;
14         ans=generate(1,n);
15         return ans;
16     }
17     vector<TreeNode*> generate(int a,int b)
18     {
19         vector<TreeNode*> ans;
20         if(a>b)
21             return ans;
22         if(a==b)
23         {
24             TreeNode *temp=new TreeNode(a);
25             ans.push_back(temp);
26             return ans;
27         }
28         vector<TreeNode*> vt1=generate(a+1,b);
29         for(int i=0;i<vt1.size();i++)
30         {
31             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）
32             tr1->right=vt1[i];
33             ans.push_back(tr1);
34         }
35         
36         vector<TreeNode*> vt2=generate(a,b-1);
37         for(int i=0;i<vt2.size();i++)
38         {
39             TreeNode *tr2=new TreeNode(b);
40             tr2->left=vt2[i];
41             ans.push_back(tr2);
42         }
43         
44         for(int i=a+1;i<=b-1;i++)
45         {
46             vector<TreeNode*> vt31=generate(a,i-1);
47             vector<TreeNode*> vt32=generate(i+1,b);
48             for(int j=0;j<vt31.size();j++)
49             {
50                 for(int k=0;k<vt32.size();k++)
51                 {
52                     TreeNode *tr3=new TreeNode(i);
53                     tr3->left=vt31[j];
54                     tr3->right=vt32[k];
55                     ans.push_back(tr3);
56                 }
57             }
58             
59             
60         }
61         return ans;
62     }
63 };

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