Codility - Odd occurrences

odd-occurrences.py

def solution(A):
    A = sorted(A)
    for index in range(0, len(A), 2):
        next_index = index+1
        if next_index >= len(A) or A[index] != A[index+1]:
            return A[index]

solution-2.py

def solution(A):
    result = A[0]
    for index in range(1, len(A)):
        result = result ^ A[index]
    return result

solution-3.py

def solution(A):
    return reduce(lambda x, y: x^y, A)