974. Subarray Sums Divisible by K

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Problem

Given an integer array nums and an integer k, return the number of non-empty subarrays that have a sum divisible by k.

A subarray is a contiguous part of an array.

Example 1:

Input: nums = [4,5,0,-2,-3,1], k = 5
Output: 7
Explanation: There are 7 subarrays with a sum divisible by k = 5:
[4, 5, 0, -2, -3, 1], [5], [5, 0], [5, 0, -2, -3], [0], [0, -2, -3], [-2, -3]

Example 2:

Input: nums = [5], k = 9
Output: 0

Code

class Solution {
    public int subarraysDivByK(int[] nums, int k) {
        HashMap<Integer, Integer> map = new HashMap<>();
        int sum = 0;
        int res = 0;
        map.put(0, 1);
        for (int num : nums) {
            sum = (sum + num % k + k) % k;
            if (!map.containsKey(sum)) {
                map.put(sum, 0);
            }
            res += map.get(sum);
            map.put(sum, map.get(sum) + 1);
        }

        return res;
    }
}