LeetCode Unique Binary Search Trees II

class Solution {
private:
    vector<TreeNode *> res;
public:
    vector<TreeNode *> generateTrees(int n) {
        res.clear();
        res = dfs(1, n + 1);
        return res;
    }

    vector<TreeNode*> dfs(int start, int end) {
        vector<TreeNode*> res;
        if (start >= end) {
            res.push_back(NULL);
            return res;
        }
        TreeNode* rt = NULL;
        for (int i=start; i<end; i++) {
            vector<TreeNode*> lsub = dfs(start, i);
            vector<TreeNode*> rsub = dfs(i+1, end);

            for (int li=0; li<lsub.size(); li++) {
                for (int ri=0; ri<rsub.size(); ri++) {
                    rt = new TreeNode(i);
                    rt->left = lsub[li];
                    rt->right= rsub[ri];
                    res.push_back(rt);
                }
            }
        }
        return res;
    }
};