LeetCode "Unique Binary Search Trees II"

I fell in a deadloop, and didn't see the actual recursion pattern. 

Reference:http://fisherlei.blogspot.com/2013/03/leetcode-unique-binary-search-trees-ii.html

class Solution {
public:
    vector<TreeNode *> generate(int s, int e)
    {
        vector<TreeNode *> subTree; 
        if(s > e)    
        {
            subTree.push_back(NULL);
            return subTree;
        }

        for(int i = s; i <= e; i++)  
                {
            vector<TreeNode*> leftSubs = generate(s, i-1);  
            vector<TreeNode*> rightSubs = generate(i+1, e);  
            for(int j = 0; j < leftSubs.size(); j ++)
            for(int k = 0; k < rightSubs.size(); k ++)
            {
                TreeNode *pRoot = new TreeNode(i);
                pRoot->left = leftSubs[j];
                pRoot->right= rightSubs[k];
                subTree.push_back(pRoot);
            }
        }
        return subTree;
    }
    vector<TreeNode *> generateTrees(int n) {
        if(n == 0) return generate(1,0);
        return generate(1, n);
    }
};