Create a bool array " prime [ 0 . . N ] " and initialize all the entries in it as true . A value in prime [ i ] will finally be false if i is Not a prime , else true .
bool [ ] check = new bool [ N + 1 ] ; for ( int i = 0 ; i <= N ; i ++ ) check [ i ] = true ; for ( int p = 2 ; p * p <= N ; p ++ ) {