95.Unique Binary Search Trees II

/**
 * 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) {
        if(n<1){
            vector<TreeNode *> result;
            return result;
        }
        return help(1,n);
    }
    vector<TreeNode *> help(int start,int end){
        vector<TreeNode *> result;
        if(start>end){
            result.push_back(NULL);
            return result;
        }
        for(int i=start;i<=end;i++){
            vector<TreeNode *> left =help(start,i-1);
            vector<TreeNode *> right=help(i+1,end);
            for(int j=0;j<left.size();j++){
                for(int k=0;k<right.size();k++){
                    TreeNode *root =new TreeNode(i);
                    root->left=left[j];
                    root->right=right[k];
                    result.push_back(root);
                    
                }
            }
        }
        return result;
        
        
    }
};