95. Unique Binary Search Trees II LeetCode

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<TreeNode*> dfs(int l,int r)
{
vector<TreeNode*> ans;
TreeNode* root = NULL;
if(l > r)
{
ans.push_back(root);
return ans;
}
for(int i = l; i <= r; i++)
{
vector<TreeNode*> left = dfs(l,i - 1);
vector<TreeNode*> right = dfs(i + 1,r);
for(int j = 0; j < left.size(); j++)
{
for(int k = 0; k < right.size(); k++)
{
root = new TreeNode(i);
root->left = left[j];
root->right = right[k];
ans.push_back(root);
}
}
}
return ans;
}
vector<TreeNode*> generateTrees(int n) {
vector<TreeNode*> ans;
ans.clear();
if(n == 0) return ans;
ans = dfs(1,n);
return ans;
}
};