77. Combinations

Medium

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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

Solution:

class Solution {
    private void f(int cur,int n,List<Integer> no,List<List<Integer>> ans,int k){
        if(no.size() == k){
            List<Integer> res = new ArrayList<>(no); 
            ans.add(res);
            return;
        }for(int i=cur;i<=n;i++) {
            no.add(i);
            f(i+1,n,no,ans,k);
            no.remove(no.size()-1);
        }


    }
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> ans = new ArrayList<>();
        List<Integer> no = new ArrayList<>();
        f(1,n,no,ans,k);
        return ans;
    }
}