HackerRank 'Sherlock and Pairs' Solution

Martin Kysel · March 24, 2015

Short Problem Definition:

Sherlock is given an array of N integers A0, A1 … AN-1 by Watson. Now Watson asks Sherlock how many different pairs of indices i and j exist such that i is not equal to j but Ai is equal to Aj.

That is, Sherlock has to count total number of pairs of indices (i, j) where Ai = Aj AND i ≠ j.

Sherlock and Pairs


time complexity is O(n)

space complexity is O(n)


The first step is to create a count of all integers. Next there are choose(k,2)*2 distinct pairs for each integer count (this step similar to Handshake, but you count i,j and j,i as two distinct pairs). Count those together and you arrive at the answer.


def cnt_equals(arr):
    the_map = {}
    final_cnt = 0
    for value in arr:
        if value not in the_map:
            the_map[value] = 1
            the_map[value] += 1

    for value in the_map.values():
        if value != 1:
            final_cnt += (value*(value-1))

    return final_cnt

if __name__ == '__main__':
    t = input()
    for _ in xrange(t):
        n = input()
        arr = map(int, raw_input().split())
        print cnt_equals(arr)

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