Unique Binary Search Trees II

Example:

Input: 3
Output:
[
  [1,null,3,2],[3,2,1],1,[2,3],[1,3]
]
Explanation:
The above output corresponds to the 5 unique BST‘s shown below:

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

class Solution
{
public:
    vector<TreeNode*> generateTrees(int n)
    {
        if(n<=0)
            return {};

        return generateTreesCore(1,n);
    }
private:
    vector<TreeNode *> generateTreesCore(int start,int end)
    {
        if(start==end)
            return {new TreeNode(start)};
        else if(start>end)
            return {nullptr};

        vector<TreeNode *> res;
        for(int i=start;i<=end;++i)
        {
            auto left=generateTreesCore(start,i-1);
            auto right=generateTreesCore(i+1,end);

            for(auto &l:left)
            {
                for(auto &r:right)
                {
                    TreeNode *t=new TreeNode(i);
                    t->left=l;
                    t->right=r;
                    res.emplace_back(t);
                }
            }
        }
        return res;
    }
};