vector<vector<int> > combine(int n, int k) {
ret.clear();
vector<int> ans;
ans.clear();
combineHelper(ans, n, k, 1);
return ret;
}
vector<vector<int> > combine(int n, int k) {
vector<int> subans;
vector<vector<int>> ans;
if(n < k)
return ans;
combineHelper(ans, subans, 1 , n, k);
return ans;
}
vector<vector<int> > combine(int n, int k) {
// Note: The Solution object is instantiated only once and is reused by each test case.
vector<vector<int> > result;
vector<int> oneSolution;
combineHelper(k, n, oneSolution, result,1);
return result;
}
void combineHelper(int k, int n, vector<int> s, vector<vector<int> > &r,int index){
//base
if (k==0){
r.push_back(s);
return;
}
if (index <= n)
{
//choose current one:
s.push_back(index);
combineHelper(k-1, n, s, r, index+1);
//or not choose.
s.pop_back();
combineHelper(k, n, s, r, index+1);
}
}
void combineHelper(vector<int> &ans, int n, int k,int index) {
if(k == 0) {
ret.push_back(ans);
return;
}
int i;
for(i = index;i <= n;i ++) {
ans.push_back(i);
combineHelper(ans,n,k-1,i+1);
ans.pop_back();
}
}
void combineHelper(vector<vector<int>> &ans, vector<int>& subans, int i, int n, int k){
if(k == 0){
ans.push_back(subans);
return;
}
if(k > n - i + 1){
return;
}
for(;i <= n; i++){
subans.push_back(i);
combineHelper(ans, subans, i + 1, n, k - 1);
subans.pop_back();
}
}
