77. Combinations (Medium)

https://leetcode.com/problems/combinations/

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

Example:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

Hints

Solutions

  • Java
class Solution {
    public List<List<Integer>> combine(int n, int k) {
        int[] subArr = new int[k];
        List<List<Integer>> result = new ArrayList<>();
        getNext(0, 0, n, k, subArr, result);
        return result;
    }

    private void getNext(int i, int count, int n, int k, int[] subArr, List<List<Integer>> result) {
        if (k == 0) {
            List<Integer> subList = new ArrayList<>();
            for (int a : subArr)
                subList.add(a);
            result.add(subList);

            return;
        }

        for (int j = i + 1; j <= n; j++) {
            subArr[count] = j;
            getNext(j, count + 1, n, k - 1, subArr, result);
        }
    }
}
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