筛选法 求素数 一个数的所有因素和

/*
---------------------------------------
素数筛选法
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*/

#include<stdio.h>
int x[20000000];
int main()
{

int temp, n, i;
for( i = 0; i < 20000000; i ++ )
x[i] = 0;
x[i] = x[1] = 1;
for( i = 2; i < 20000000; i ++ )
{
if( x[i] == 0 )
{
temp = 2 * i;
while( temp <= 20000000)
{
x[temp] = 1;
temp += i;
}
}
}
n = 19880100;
while( n ++ && n < 19891235 )
{
if( x
== 0  && n % 10000 < 1235)
printf("%d\n", n);

}
return 0;
}

/*-------------------------------------------------筛选法求一个数因数的和（如12：1，2，3，4，6）-------------------------------------------------*/#include<stdio.h>#include<string.h>long long num[500005];void shaixuan( ){for( int i = 1; i < 500005; ++i )num[i] = 1;for( int i = 2; i < 500005; ++i )for( int j = 2; j * i < 500005; ++j )num[i*j] += i;}int main( ){shaixuan( );int t,n;scanf( "%d",&t );while( t-- ){scanf( "%d",&n );printf( "%I64d\n",num);}return 0;}