In C language, integer constants can be expressed in decimal, octal and hexadecimal: only numbers from 0 to 9 can appear, and they can be signed and signed. For example, 1 1

Octal integer constant: an octal digit string starting with the number 0, where the number is 0 ~ 7. For example, 0 1 1 (decimal 9)

Hexadecimal integer constant: Hexadecimal digit string starting with 0X or 0x, where each digit can be 0 ~ 9, a ~ f or a ~

Numbers or English letters in F. For example, 0x 1 1 (decimal 17)