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
vector<TreeNode *> res;
if (n <= 0) {
res.push_back(NULL);
return res;
}
generate(res, 1, n + 1);
return res;
}
void generate(vector<TreeNode *> &res, int l, int r) {
if (l == r) {
res.push_back(NULL);
return;
}
for (int i = l; i < r; ++i) {
vector<TreeNode *> leftres;
generate(leftres, l, i);
vector<TreeNode *> rightres;
generate(rightres, i + 1, r);
for (int j = 0; j < leftres.size(); ++j) {
for (int k = 0; k <rightres.size(); ++k) {
TreeNode *root = new TreeNode(i);
root->left = leftres[j];
root->right = rightres[k];
res.push_back(root);
}
}
}
}
};