74：Unique Binary Search Trees II

题目：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.
题目具体形式见 https://leetcode.com/problems/unique-binary-search-trees-ii/?tab=Description

下面解法代码的思想及编写参考了网址https://github.com/soulmachine/leetcode#leetcode题解题目，用的是递归方法
，代码如下：

class Solution {public:        vector<TreeNode*> generateTrees(int n) {                if (n == 0) return vector<TreeNode*>();                return generate(1, n);        }private:        vector<TreeNode*> generate(int start, int end) {                vector<TreeNode*> subTree;                if (start > end) {                        subTree.push_back(nullptr);                        return subTree;                }                for (int k = start, k <= end; ++k) {                        vector<TreeNode*> leftSubs = generate(start, k - 1);                        vector<TreeNode*> rightSubs = generate(k + 1, end);                        for (auto i : leftSubs)                                for (auto j : rightSubs) {                                        TreeNode* node = new TreeNode(k);                                        node -> left = i;                                        node -> right = j;                                        subTree.push_back(node);                                }                }                return subTree;        }};