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  1. Prepare
  2. Algorithms
  3. Implementation
  4. Divisible Sum Pairs
  5. Discussions

Divisible Sum Pairs

  • Jlookup
     + 8 comments

    Thanks @usinha02 for the algo, and @petreeftime for the explanation. Here's an implementation in Python. Some bits are a little clunky. Feedback Please.

    Python 3:

    import sys


n,k = [int(x) for x in input().strip().split(' ')]
a = [int(a_temp) for a_temp in input().strip().split(' ')]

mods = [0] * k
for i in range(n):
    mods[a[i] % k] += 1 

count = 0
for i in range(int(k/2) + 1):
    if i == 0:
        count += int(mods[0] * (mods[0] - 1) / 2)
    elif float(i) == (k/2):
        count += int(mods[int(k/2)] * (mods[int(k/2)] - 1) / 2)
    else:
        count += int(mods[i] * mods[k-i])

print (count)