(數論2.1.2)UVA 10168Summation of Four Primes(歐拉篩法)

/* * UVA_10168.cpp * *  Created on: 2013年10月7日 *      Author: Administrator */#include <iostream>#include <cstdio>#include <algorithm>#include <cstring>/** * 2 2 3 172 2 3 292 2 5 37 */using namespace std;int su[10000002];bool u[10000002];int num = 0;int n;void prepare(){int i,j;memset(u,true,sizeof(u));for(i = 2 ; i <= 10000002 ;++i){if(u[i]){su[++num] = i;}for(j = 1 ; j <= num ; ++j ){if(i*su[j] >10000002){break;}u[i*su[j]] = false;if(i % su[j] ==0){break;}}}}int main(){prepare();while(scanf("%d",&n)!=EOF){if(n < 8){printf("Impossible.\n");continue;}if(n == 8){printf("2 2 2 2\n");continue;}if(n == 9){printf("2 2 2 3\n");continue;}if(n == 10){printf("2 2 3 3\n");continue;}if(n == 11){printf("2 3 3 3\n");continue;}if(n == 12){printf("3 3 3 3\n");continue;}if(n % 2 == 0){printf("2 2 ");n -= 4;}else{printf("2 3 ");n -=5;}int i;for(i = 1 ; i <= num ; ++i){if(su[i] * 2 > n){break;}if(u[n - su[i]]){printf("%d %d\n",su[i],n-su[i]);break;}}}return 0;}