974. Subarray Sums Divisible by K 

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class Solution {
    public int subarraysDivByK(int[] nums, int k) {
        int sum = 0, res = 0;
        Map<Integer, Integer> remainderToCountMap = new HashMap<>();
        remainderToCountMap.put(0, 1);
        for (int num : nums) {
            sum += num;
            int remainder = ((sum % k) + k) % k; // 此处注意处理负数的余数为负的情况
            int sameCount = remainderToCountMap.getOrDefault(remainder, 0);
            res += sameCount;
            remainderToCountMap.put(remainder, sameCount + 1);
        }

        return res;
    }
}

References

974. Subarray Sums Divisible by K