Template_ Judging prime_ Square root / six prime method

//  prescribing 
bool f( int n )
{
	if( n<=3 ) return n>1;
	
    for( int i=2;i<=n/i;i++ )
        if( n%i==0 ) return false;
    return true;
}

//  Six prime method 
bool f( int n )
{
    if( n<=3 ) return n>1;
    if( n%2==0 || n%3==0 ) return false;
    
    for( int i=5;i<=n/i;i+=6 )
        if( n%i==0 || n%(i+2)==0 ) return false;
    return true;
}