实现函数exp(base, expo)，求base的expo次方。不得使用库函数，同时不需要考虑大数问题。
def exp(base, expo):
if expo == 0: return 1
elif expo == 1: return base
elif expo < 0:
return 1 / exp(base, -expo)
# if expo % 2 == 0:
# return exp(base, expo / 2) * exp(base, expo / 2)
# elif expo % 2 == 1:
# return exp(base, (expo - 1) / 2) * exp(base, (expo - 1) / 2) * exp(base, 1)
result = exp(base, expo >> 1)
result *= result
if expo & 0x1 == 1:
result *= base
return result
if __name__ == '__main__':
assert exp(3, -1) == 0.3333333333333333
assert exp(3, 10) == 59049
assert exp(2, -3) == 0.125
assert exp(2, 0) == 1
assert exp(0, 0) == 1
assert exp(0, 1) == 0
assert exp(3, 0) == 1
