Gets the index of the dominator of array A

array_dominator.py

from collections import Counter

def solution(A):
    """Gets the index of the dominator of array A"""
    counter = Counter()

    for i, v in enumerate(A):
        counter[v] += 1

    counter = counter.most_common()

    if len(counter) == 0:
        return -1
    if len(counter) == 1:
        return A.index(counter[0][0])
    elif counter[0][1] > counter[1][1]:
        return A.index(counter[0][0])

    return -1


if __name__ == '__main__':
    assert solution([2, 2, 3, 3, 1]) == -1
    assert solution([2, 2, 3, 1]) == 0