LeetCode 95: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

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

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//递归的方法
class Solution{
public: 
	vector<TreeNode*> generateTrees(int n){
		return generate(1, n);
	}

	vector<TreeNode*> generate(int begin, int end){
		vector<TreeNode*> res;
		if (begin > end)   res.push_back(NULL);
		if (begin == end)  res.push_back(new TreeNode(begin));
		if (begin < end)
		{
			for (int k = begin; k <= end; k++){
				vector<TreeNode*> l = generate(begin, k - 1);
				vector<TreeNode*> r = generate(k + 1, end);
				for (int i = 0; i < l.size(); i++)
				{
					for (int j = 0; j < r.size(); j++)
					{
						TreeNode* node = new TreeNode(k);
						node->left = l[i];
						node->right = r[j];
						res.push_back(node);
					}
				}
			}
		}
		return res;
	}
};