Algo: brout-force enumeration + bit operation.

Note: It doesn't fit large n but small k. Admin can add this type of test case.

```
class Solution {
public:
vector<vector<int> > combine(int n, int k) {
vector< vector<int> > ans;
for (int mask = 0; mask < (1 << n); ++mask) {
int cnt = 0;
for (int j = 0; j < n; ++j) if (mask & (1 << j)) ++cnt;
if (cnt == k) {
vector<int> tmp;
for (int j = 0; j < n; ++j) if (mask & (1 << j)) {
tmp.push_back(j + 1);
}
ans.push_back(move(tmp));
}
}
return ans;
}
};
```