leetcode 974. Subarray Sums Divisible by K

Another Two Sum

class Solution {
    public int subarraysDivByK(int[] A, int K) {
        int[] sum = new int[A.length + 1];
        for (int i = 0; i < A.length; ++i) sum[i + 1] = sum[i] + A[i];
        Map<Integer, Integer> c = new HashMap<Integer, Integer>();
        int ret = 0;
        for (int i = 0; i <= A.length; ++i) {
            int v = (sum[i] % K + K) % K;
            int count = c.getOrDefault(v, 0);
            ret += count;
            c.put(v, count + 1);
        }
        return ret;
    }
}