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 <= 20
- 1 <= k <= n
class Solution {
public List<List<Integer>> combine(int n, int k) {
List<List<Integer>> res = new ArrayList<>();
backtrack(res, new ArrayList(), n, k,1);
return res;
}
private void backtrack(List<List<Integer>> res, List<Integer> tempList, int n, int k,int start){
if(tempList.size()==k){
res.add(new ArrayList(tempList));
return;
}
for(int i=start; i<=n; i++){
tempList.add(i);
backtrack(res,tempList,n,k,i+1);
tempList.remove(tempList.size()-1);
}
}
}