Given an array A
of integers, return the number of (contiguous, non-empty) subarrays that have a sum divisible by K
.
Example 1:
Input: A = [4,5,0,-2,-3,1], K = 5
Output: 7
Explanation: There are 7 subarrays with a sum divisible by K = 5:
[4, 5, 0, -2, -3, 1], [5], [5, 0], [5, 0, -2, -3], [0], [0, -2, -3], [-2, -3]
Note:
1 <= A.length <= 30000
-10000 <= A[i] <= 10000
2 <= K <= 10000
Idea 1. prefix sums + HashMap + modular rules, note: count[0] = 1, (X + X%K)%K for negative values
Time complexity: O(n)
Space complexity: O(K)
1 class Solution { 2 public int subarraysDivByK(int[] A, int K) { 3 int[] count = new int[K]; 4 count[0] = 1; 5 int prefixSum = 0; 6 7 int result = 0; 8 for(int a: A) { 9 prefixSum += a; 10 prefixSum = (prefixSum % K + K)%K; 11 result += count[prefixSum]; 12 ++count[prefixSum]; 13 } 14 15 return result; 16 } 17 }
Idea 1.a count pairs
1 class Solution { 2 public int subarraysDivByK(int[] A, int K) { 3 int[] count = new int[K]; 4 count[0] = 1; 5 int prefixSum = 0; 6 7 for(int a: A) { 8 prefixSum += a; 9 prefixSum = (prefixSum % K + K)%K; 10 ++count[prefixSum]; 11 } 12 13 int result = 0; 14 for(int val: count) { 15 result += val*(val-1)/2; 16 } 17 18 return result; 19 } 20 }