LeetCode: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

见上面一题

 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        if(n==0) return generate(1,0);
14         return generate(1,n);
15     }
16     
17     vector<TreeNode *> generate(int start,int end)
18     {
19         vector<TreeNode *> subtree;
20         if(start>end)
21         {
22          subtree.push_back(NULL);
23          return subtree;
24          
25         }
26         
27         for(int k=start;k<=end;k++)
28         {
29             vector<TreeNode *> leftsubs=generate(start,k-1);
30             vector<TreeNode *> rightsubs=generate(k+1,end);
31             
32             for(auto i:leftsubs)
33                 for(auto j:rightsubs)
34                 {
35                     TreeNode *node=new TreeNode(k);
36                     node->left=i;
37                     node->right=j;
38                     subtree.push_back(node);
39                 }
40         }
41         return subtree;
42     }
43     
44     
45 };