Sieve of Eratosthenes
static void sieve ( ) { Arrays . fill ( primes , true ) ; primes [ 0 ] = false ; primes [ 1 ] = false ; for ( int i = 2 ; i * i <= maxn ; i ++ ) { if ( primes [ i ] ) { for ( int j = i * i ; j <= maxn ; j += i ) primes [ j ] = false ; } } }