LeetCode-Unique Binary Search Trees II

生成二叉查找树；

哎，思考了大半天才写出来，

主要卡在生成结点上了，开始没考虑好怎么把不同的树在不同的地方复制前面的部分，

还是对递归的使用不够成熟，理解和掌握的还不够好！

 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         // Start typing your C/C++ solution below
14         // DO NOT write int main() function
15         vector<TreeNode *> res;
16         if (n <= 0) {
17             res.push_back(NULL);
18             return res;
19         }
20         generate(res, 1, n + 1);
21         return res;
22     }
23     void generate(vector<TreeNode *> &res, int l, int r) {
24         if (l == r) {
25             res.push_back(NULL);
26             return;
27         }
28         for (int i = l; i < r; ++i) {
29             vector<TreeNode *> leftres;
30             generate(leftres, l, i);
31             vector<TreeNode *> rightres;
32             generate(rightres, i + 1, r);
33             for (int j = 0; j < leftres.size(); ++j) {
34                 for (int k = 0; k <rightres.size(); ++k) {
35                     TreeNode *root = new TreeNode(i);
36                     root->left = leftres[j];
37                     root->right = rightres[k];
38                     res.push_back(root);
39                 }
40             }
41         }
42     }
43 };