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974. Subarray Sums Divisible by K 👍

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class Solution {
 public:
  int subarraysDivByK(vector<int>& nums, int k) {
    int ans = 0;
    int prefix = 0;
    vector<int> count(k);
    count[0] = 1;

    for (const int num : nums) {
      prefix = (prefix + num % k + k) % k;
      ans += count[prefix];
      ++count[prefix];
    }

    return ans;
  }
};

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class Solution {
  public int subarraysDivByK(int[] nums, int k) {
    int ans = 0;
    int prefix = 0;
    int[] count = new int[k];
    count[0] = 1;

    for (final int num : nums) {
      prefix = (prefix + num % k + k) % k;
      ans += count[prefix];
      ++count[prefix];
    }

    return ans;
  }
}

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class Solution:
  def subarraysDivByK(self, nums: list[int], k: int) -> int:
    ans = 0
    prefix = 0
    count = [0] * k
    count[0] = 1

    for num in nums:
      prefix = (prefix + num % k + k) % k
      ans += count[prefix]
      count[prefix] += 1

    return ans