LeetCode 77. Combinations

Question

Given two integers n and k, return all possible combinations of k numbers out of the range [1, n].

You may return the answer in any order.

https://leetcode.com/problems/combinations

• Solution1

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

Complexity:

Time complexity: O(C(n, k))

Space complexity: O(k)