What if I don't write default in switch case?

The code is valid. If there is no default: label and none of the case labels match the "switched" value, then none of the controlled compound statement will be executed. Execution will continue from the end of the switch statement.

ISO/IEC 9899:1999, section 6.8.4.2:

[...] If no converted case constant expression matches and there is no default label, no part of the switch body is executed.

As others have pointed out it is perfectly valid code. However, from a coding style perspective I prefer adding an empty default statement with a comment to make clear that I didn't unintentionally forget about it.

int a=10;
switch(a)
{
case 0: printf("case 0");
         break;
case 1: printf("case 1");
         break;
default: // do nothing;
         break;
}

The code generated with / without the default should be identical.

It is perfectly legal code. If a is neither 0 or 1, then the switch block will be entirely skipped.