0668 is not a legal integer constant. 0 starts with octal, and only the numbers 0-7 can appear. There are 8 in 0668, so it is not a legal number.

Integer constants are divided into decimal integer constants, octal integer constants and hexadecimal integer constants.

(1) decimal integer constant This constant can only have numbers from 0 to 9, and can also have positive and negative signs.

(2) Octal integer constant This constant is an octal digit string starting with the number 0. Where the numbers are 0 ~ 7.

(3) Hexadecimal integer constant This constant is a hexadecimal digit string starting with 0x or 0x. Where each number can be

Numbers or English letters in 0 ~ 9, a ~ f or a ~ f.