BestCoder#51

 #include <cstdio>
 #include <iostream>
 #include <cmath>
 #include <cstring>
 using namespace std;
 bool visit[];
 int prime[];

 void init_prim()
 {
     memset(visit, true, sizeof(visit));
     int num = ;
     for (int i = ; i <= ; ++i)
     {
         if (visit[i] == true)
         {
             num++;
             prime[num] = i;
         }
         for (int j = ; ((j <= num) && (i * prime[j] <= ));  ++j)
         {
             visit[i * prime[j]] = false;
             if (i % prime[j] == ) break;
         }
     }
 }
 int main() {
     memset(prime, , sizeof(prime));
     int T, n;
     init_prim();
     scanf("%d", &T);
     while(T--) {
         scanf("%d", &n);
         bool flag = true;
         if(n < 1e6) {
             if(n == ) {
                 printf("%d\n", );
             } else if(visit[n] == true){
                 printf("%d\n", n-);
             } else {
                 printf("%d\n", );
             }
         } else {
             for(int i = ; prime[i] <= sqrt(n); ++i) {
                 if(n % prime[i] == ) {
                     flag = false;
                     printf("%d\n", );
                     break;
                 }
             }
             if(flag) {
                 printf("%d\n", n-);
             }
         }
     }
     return ;
 }

第一题