LeetCode: Unique Binary Search Trees II

class Solution {
public:
vector<TreeNode *> solve(vector<int> &num,int start,int n){
vector<TreeNode *> ret;
if(n == 0){
ret.push_back(NULL);
return ret;
}
for(int i = start;i < start + n;i++){
int nLeft = i - start;
int nRight = n - nLeft - 1;

vector<TreeNode *> leftRet, rightRet;
leftRet = solve(num,start,nLeft);
rightRet = solve(num,i+1,nRight);
for(int j = 0;j < leftRet.size();j++){

for(int k = 0;k < rightRet.size();k++){
TreeNode * root = new TreeNode(num[i]);
root->left = leftRet[j];
root->right = rightRet[k];
ret.push_back(root);
}
}
}
return ret;
}
vector<TreeNode *> generateTrees(int n) {
vector<int> num(n,0);
for(int i = 0;i <n;i++)
num[i] = i+1;
vector<TreeNode *> ret;
ret = solve(num,0,n);
return ret;
}
};