【LeetCode】Unique Binary Search Trees II

/**
 * Definition for binary tree
 * 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) {
        vector<TreeNode *> res;
        if(n == 0)
        {
            res.push_back(NULL);
            return res;
        }
        else 
        {
            return Helper(1, n);
        }
    }
    vector<TreeNode*> Helper(int begin, int end)
    {
        vector<TreeNode*> res;
        if(begin > end)
        {
            res.push_back(NULL);
            return res;
        }
        else if(begin == end)
        {
            TreeNode* root = new TreeNode(begin);
            res.push_back(root);
            return res;
        }
        else
        {
            for(int i = begin; i <= end; i ++)
            {
                //tag
                vector<TreeNode*> left = Helper(begin, i-1);
                vector<TreeNode*> right = Helper(i+1, end);
                for(int l = 0; l < left.size(); l ++)
                {//all BST in left
                    for(int r = 0; r < right.size(); r ++)
                    {//all BST in right
                        //new root here! not the tag position.
                        TreeNode* root = new TreeNode(i);
                        root->left = left[l];
                        root->right = right[r];
                        res.push_back(root);
                    }
                }
                
            }
            return res;
        }
    }
};