Q: https://leetcode.com/problems/subarray-sums-divisible-by-k/
int n = nums.length;
int prefixMod = 0, result = 0;
int[] modGroups = new int[k];
modGroups[0] = 1;
for (int num: nums) {
prefixMod = (prefixMod + num % k + k) % k;
result += modGroups[prefixMod];
modGroups[prefixMod]++;
}
return result;
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