lc95+Unique Binary Search Trees II

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

class Solution {
public:
    vector<TreeNode*>generate(int start, int end)
    {
        vector<TreeNode*>ret;
        if(start>end){
            ret.push_back(NULL);
            return ret;
        }
        for(int i=start;i<=end;i++){
            vector<TreeNode*>lTree=generate(start,i-1);
            vector<TreeNode*>rTree=generate(i+1,end);
            for(int j=0;j<lTree.size();j++){
                for(int k=0;k<rTree.size();k++){
                    TreeNode* node=new TreeNode(i+1);
                    ret.push_back(node);
                    node->left=lTree[j];
                    node->right=rTree[k];
                }
            }
        }
        return ret;
    }
    vector<TreeNode*> generateTrees(int n) {
        vector<TreeNode*>ret;
        if(n<=0) return ret;
        return generate(0,n-1);
    }
};