Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].
You may return the answer in any order.
Example 1:
Input: n = 4, k = 2 Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]] Explanation: There are 4 choose 2 = 6 total combinations. Note that combinations are unordered, i.e., [1,2] and [2,1] are considered to be the same combination.
Example 2:
Input: n = 1, k = 1 Output: [[1]] Explanation: There is 1 choose 1 = 1 total combination.
Constraints:
1 <= n <= 201 <= k <= nclass Solution {
public:
vector<vector<int>> res;
void solve(int start, int n, int k, vector<int> v = {}){
if(v.size()==k)
res.push_back(v);
else if(v.size()+n-start+1<k);
else
for(int i=start; i<=n; i++){
v.push_back(i);
solve(i+1, n, k, v);
v.pop_back();
}
}
vector<vector<int>> combine(int n, int k) {
solve(1, n, k);
return res;
}
};