在Python中,当一个整数大于2**31时,它将会变成长整型:
a = 2147483647
a + 1 = 2147483648
b = -2147483648
b - 1 = -2147483649
a = 2147483647
a + 1 = -2147483648
b = -2147483648
b - 1 = 2147483647