Large integer class
On a 32-bit operating system, the length of an integer is 32 bits, that is, 4 bytes, which is enough for general applications. But in some fields, such as cryptography, it is often necessary to use integers with the length of 128 bits (16 bytes). For this integer, the internal data type cannot be processed, so it is necessary to implement such a large integer class. And use this class to implement related algorithms.