[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

由于要列出所有解，所有就只能用DFS枚举了。

 1 /**
 2  * Definition for binary tree
 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 *> generate(int beg, int end)
13     {
14         vector<TreeNode* > ret;
15         if (beg > end)
16         {
17             ret.push_back(NULL);
18             return ret;
19         }
20         
21         for(int i = beg; i <= end; i++)
22         {
23             vector<TreeNode* > leftTree = generate(beg, i - 1);
24             vector<TreeNode* > rightTree = generate(i + 1, end);
25             for(int j = 0; j < leftTree.size(); j++)
26                 for(int k = 0; k < rightTree.size(); k++)
27                 {
28                     TreeNode *node = new TreeNode(i + 1);
29                     ret.push_back(node);
30                     node->left = leftTree[j];
31                     node->right = rightTree[k];              
32                 }           
33         }
34         
35         return ret;
36     }
37     
38     vector<TreeNode *> generateTrees(int n) {
39         // Start typing your C/C++ solution below
40         // DO NOT write int main() function
41         return generate(0, n - 1);
42     }
43 };