I am very happy because I solved this problem with very little code:

"""

    100 people are standing in a circle with gun in their hands.
    1 kills 2, 3 kills 4, 5 kills 6 and so on till we are
    left with only one person. Who will be the last person alive?
    Write code to implement this ##efficiently.## <-[ Python is not efficient]

"""

persons = list(range(1,101)) # The question asks 1-indexing

while len(persons) > 1:
    for index, person in enumerate(persons):
        del persons[(index + 1) % len(persons)]

print(persons)
    

@dotancohen here at CodeReview we review different implementations of the same algorithm, also that question is in Java, a different language.