hdu2710 筛选法求素数

#include <iostream>
#include <string>
#include <cstring>
#include <queue>
#include <stack>
#include <cmath>
#include <algorithm>
#include <cstdio>

using namespace std;

bool prime[20001];
int max_prime[20001];
int a[20001];
int b[20001];

void is_prime()
{
prime[0] = false;
prime[1] = false;
for(int i = 2 ; i * i <= 20000 ; i++ )
{
if(prime[i])
{
int j = 2 * i;
while(j <= 20000)
{
prime[j] = false ;
j += i;
}
}
}
}

void is_max_prime()
{
max_prime[0] = 0;
max_prime[1] = 0;
for(int i = 2 ; i <= 20000 ; i ++ )
{
for(int j = i ; j >= 2 ; j--)
{
if(prime[j] && i % j == 0)
{
max_prime[i] = j;
break;
}
//  if(j == i ) j /= 2;
}
}
}

int main()
{
int n;
memset(prime , true , sizeof(prime));
is_prime();
is_max_prime();
while(cin >> n)
{
for(int i = 0 ;i < n ; i ++ )
{
cin >> a[i];
b[i] = max_prime[a[i]];
}
int num = -1 ;
int res ;
for(int i = 0 ; i < n ; i ++ )
{
if(b[i] > num )
{
num = b[i];
res = a[i];
}
}
cout << res <<endl;
}
}