LeetCode-Unique Binary Search Trees II

/**
* Definition for binary tree
* 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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
return generate(1, n);
}

vector<TreeNode *> generate(int beg, int end)
{
vector<TreeNode *> ret;
if (beg > end)
{
ret.push_back(NULL);
return ret;;
}
for (int i = beg; i <= end; ++i)
{
vector<TreeNode *> leftTree = generate(beg, i - 1);
vector<TreeNode *> rightTree = generate(i + 1, end);
for (int j = 0; j < leftTree.size(); ++j)
{
for (int k = 0; k < rightTree.size(); ++k)
{
TreeNode *ptr = new TreeNode(i);
ptr->left = leftTree[j];
ptr->right = rightTree[k];
ret.push_back(ptr);
}
}

}
return ret;
}
};