77. Combinations

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LeetCode

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Problem

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],
]

Code

class Solution {
    public List<List<Integer>> combine(int n, int k) {
        int[] nums = new int[n];
        for(int i = 0; i < n; i++){
            nums[i] = i + 1;
        }

        List<List<Integer>> res = new ArrayList<>();
        helper(res, nums, new ArrayList<>(), 0, k);
        return res;
    }

    public void helper(List<List<Integer>> res, int[] nums, List<Integer> temp, int index, int count){
        if(count == 0){
            res.add(new ArrayList<>(temp));
            return;
        }

        for(int i = index; i < nums.length; i++){
            temp.add(nums[i]);
            helper(res, nums, temp, i + 1, count - 1);
            temp.remove(temp.size() - 1);
        }
    }
}