#974. Subarray Sums Divisible by K

/**
 * @param {number[]} nums
 * @param {number} k
 * @return {number}
 */

var subarraysDivByK = function(nums, k) {
  let prefixMod = 0;
  let result = 0;
  const modGroups = new Array(k).fill(0);
  modGroups[0] = 1;

  for (const num of nums) {
    // Take modulo twice to avoid negative remainders.
    prefixMod = (prefixMod + num % k + k) % k;
    result += modGroups[prefixMod];
    modGroups[prefixMod]++;
  }

  return result;
}