[Leetcode]Unique Binary Search Trees II

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

这道题思路和第一题其实有些类似。第一题是利用动态规划的思想，选取一个数作为根节点，然后分别计算左子树和右子树的数量，递归计算，得出最后结果。

这道题可以选取某个节点作为根节点，然后分别生成左子树和有字数的集合，然后把根节点和左子树、右子树的集合组合起来就得到所有的BST了

class Solution {
public:
	vector<TreeNode *> generateTrees(int n) {
		return generate(1, n);

	}
	vector<TreeNode *> generate(int begin,int end){
		vector<TreeNode *>result;
		if (begin > end){
		    result.push_back(NULL);
			return result;
		}
		for (int i = begin; i <= end; i++){
			vector<TreeNode *> left = generate(begin, i - 1);
			vector<TreeNode *> right = generate( i + 1,end);
			for (int m = 0; m < left.size(); m++){
				for (int n = 0; n < right.size(); n++){
					TreeNode *newNode = new TreeNode(i);
					newNode->left = left[m];
					newNode->right = right[n];
					result.push_back(newNode);
				}
			}
		}
		return result;
	}
};