不同的二叉搜尋樹2

給定一個整數 n，生成所有由 1 ... n 為節點所組成的二叉搜尋樹。

示例:

輸入: 3

輸出:

[

  [1,null,3,2],

  [3,2,null,1],

  [3,1,null,null,2],

  [2,1,3],

  [1,null,2,null,3]

]

解釋:

以上的輸出對應以下 5 種不同結構的二叉搜尋樹：

1 3 3 2 1

\ / / / \ \

3 2 1 1 3 2

/ / \ \

2 1 2 3

 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 *> generateTrees(int
 
 n) {
13         vector<TreeNode *> ret;
14         if (n == 0)
15             return ret;
16         return Helper(1, n);
17     }
18     vector<TreeNode *> Helper(int begin, int end)
19     {
20         vector<TreeNode *> ret;
21         if (begin > end)
22             ret.push_back(NULL);
 
23         else if (begin == end)
24         {
25             TreeNode* node = new TreeNode(begin);
26             ret.push_back(node);
27         }
28         else
29         {
30             for (int i = begin; i <= end; i++)
31             {//root
32                 vector<TreeNode *> left = Helper(begin, i - 1);
33                 vector<TreeNode *> right = Helper(i + 1, end);
34                 for (int l = 0; l < left.size(); l++)
35                 {
36                     for (int r = 0; r < right.size(); r++)
37                     {
38                         //new tree
39                         TreeNode* root = new TreeNode(i);
40                         root->left = left[l];
41                         root->right = right[r];
42                         ret.push_back(root);
43                     }
44                 }
45             }
46         }
47         return ret;
48     }
49 };