Leetcode——不同的二叉搜索树 II？？

输入：3
输出：
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
解释：
以上的输出对应以下 5 种不同结构的二叉搜索树：

   1         3     3      2      1
    \       /     /      / \           3     2     1      1   3      2
    /     /       \                    2     1         2                 3

#include <iostream>
#include <vector>
using namespace std;

struct TreeNode{
	int val;
	TreeNode* left;
	TreeNode* right;
	TreeNode(int v):val(v),left(NULL),right(NULL){}
};

class Solution {
public:
    vector<TreeNode*> generateTrees(int n) {
        vector<TreeNode*> ret;
        if(n == 0) return ret;
        
        ret = reverse(1, n);
        return ret;
    }
    
    vector<TreeNode*> reverse(int start, int end){
        vector<TreeNode*> ret;
        if(start>end){
            ret.push_back(NULL);
        }
        
        cout<<"11"<<endl;
        for(int i=start; i<=end; i++){
            vector<TreeNode*> left = reverse(start, i-1);
            cout<<"~~~";
            vector<TreeNode*> right = reverse(i+1, end);
            cout<<"---";
            for(auto l : left){
                for(auto r : right){
                    TreeNode* root = new TreeNode(i);
                    root->left = l;
                    root->right = r;
                    ret.push_back(root);
                }
            }
        }
        return ret;
    }

    void print(vector<TreeNode*> root){
    	TreeNode *tmp;
    	for(int i=0; i<root.size(); i++){
    		tmp = root[i];
    	}
    }
};
int main(){
	Solution s;
	vector<TreeNode*> res = s.generateTrees(2);
	cout<<res.size()<<endl;
	cout<<res[0]->val<<" "<<res[0]->left<<" "<<res[0]->right->val<<endl;
	return 0;
}