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struct TreeNode {	int val;	TreeNode *left;	TreeNode *right;	TreeNode(int x) : val(x), left(NULL), right(NULL) {}};class Solution {public:	vector
   
     GenerateTrees(int lower, int higher)	{		vector
    
      res;		if(lower > higher)		{			res.push_back(NULL);			return res;		}		for (int i = lower; i <= higher; ++i)		{			vector
     
       leftTree = GenerateTrees(lower, i-1);			vector
      
        rightTree = GenerateTrees(i+1, higher);			for (int l = 0; l < leftTree.size(); ++l)			{				for (int r = 0; r < rightTree.size(); ++r)				{					TreeNode* root = new TreeNode(i);					root->left = leftTree[l];					root->right = rightTree[r];					res.push_back(root);				}			}		}		return res;	}	vector
       
         generateTrees(int n) {		// Start typing your C/C++ solution below		// DO NOT write int main() function		return GenerateTrees(1, n);	}};
       
      
     
    
   

second time

/** * 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
   
     generateUtil(int start, int end)    {        vector
    
      ans;        if(start > end)        {            ans.push_back(NULL);            return ans;                   }                for(int i = start; i <= end; ++i)        {            vector
     
       leftTrees = generateUtil(start, i-1);            vector
      
        rightTrees = generateUtil(i+1, end);            for(int l = 0; l < leftTrees.size(); ++l)            {                for(int r = 0; r < rightTrees.size(); ++r)                {                    TreeNode* root = new TreeNode(i);                    root->left = leftTrees[l];                    root->right = rightTrees[r];                    ans.push_back(root);                }            }        }        return ans;    }    vector
       
         generateTrees(int n) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        return generateUtil(1, n);    }};
       
      
     
    
   

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