import operator
def single_number(l):
return reduce(operator.xor, l, 0)
print single_number([1, 1, 2, 2, 3, 4, 4, 5, 5])
aW1wb3J0IG9wZXJhdG9yCgpkZWYgc2luZ2xlX251bWJlcihsKToKICAgIHJldHVybiByZWR1Y2Uob3BlcmF0b3IueG9yLCBsLCAwKQoKcHJpbnQgc2luZ2xlX251bWJlcihbMSwgMSwgMiwgMiwgMywgNCwgNCwgNSwgNV0p