LeetCode OJ ：Unique Binary Search Trees II（唯一二叉搜索树）

题目如下所示：返回的结果是一个Node的Vector：

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　　树节点的定义是下面这样的 

 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         return createNode(1, n);
14     }
15
16     vector<TreeNode*> createNode(int start, int end)
17     {
18         vector<TreeNode*> result;
19         if(start > end){
20             result.push_back(NULL);
21             return result;
22         }
23         for(int i = start; i <= end; ++i){
24             vector<TreeNode*> leftNode = createNode(start, i - 1);
25             vector<TreeNode*> rightNode = createNode(i + 1, end);
26             for(int j = 0; j < leftNode.size(); ++j){
27                 for(int k = 0; k < rightNode.size(); ++k){
28                     TreeNode * tmpNode = new TreeNode(i);
29                     tmpNode->left = leftNode[j];　　
30                     tmpNode->right = rightNode[k];
31                     result.push_back(tmpNode);
32                 }
33             }
34         }
35         return result;
36     }
37 };

这一题实际上更另外一个叫做different ways to add parentheses的题目比较相似，这个也是详见博文