leetcode95 Unique Binary Search Trees II

思路：

分治。

实现：

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode 
 4  * {
 5  *     int val;
 6  *     TreeNode *left;
 7  *     TreeNode *right;
 8  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 9  * };
10  */
11 class Solution 
12 {
13 public:
14     vector<TreeNode *> dfs(int l, int r)
15     {
16         vector<TreeNode *> ret;
17         if (l > r) 
18         {
19             ret.push_back(NULL);
20             return ret;
21         }
22         if (l == r) 
23         {
24             ret.push_back(new TreeNode(l));
25             return ret;
26         }
27         for (int i = l; i <= r; i++)
28         {
29             vector<TreeNode *> lc = dfs(l, i - 1);
30             vector<TreeNode *> rc = dfs(i + 1, r);
31             for (auto j : lc)
32             {
33                 for (auto k : rc)
34                 {
35                     TreeNode * tmp = new TreeNode(i);
36                     tmp->left = j; tmp->right = k;
37                     ret.push_back(tmp);
38                 }
39             }
40         }
41         return ret;
42     }
43     vector<TreeNode *> generateTrees(int n) 
44     {
45         if (!n) return vector<TreeNode *>();
46         return dfs(1, n);
47     }
48 };