【Unique Binary Search Trees II】cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<TreeNode*> generateTrees(int n) {
        return Solution::generateBST(1, n);
    }
        static vector<TreeNode*> generateBST(int min, int max)
        {
            vector<TreeNode*> ret;
            if ( min>max ) { ret.push_back(NULL); return ret; }
            for ( int i = min; i<=max; ++i )
            {
                vector<TreeNode*> left = Solution::generateBST(min, i-1);
                vector<TreeNode*> right = Solution::generateBST(i+1,max);
                for ( size_t l = 0; l < left.size(); ++l )
                {
                    for ( size_t r = 0; r < right.size(); ++r )
                    {
                        TreeNode *root = new TreeNode(i);
                        root->left = left[l];
                        root->right = right[r];
                        ret.push_back(root);
                    }
                }
            }
            return ret;
        }
};