zoj 1657

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素数

#include"stdio.h"#include"string.h"#define N 100000int IsPrime[N],cnt;int prime[N];void init(){int i,j;cnt=0;memset(prime,0,sizeof(prime));for(i=2;i<N;i++){if(!prime[i]){IsPrime[cnt++]=i;for(j=i+i;j<N;j+=i)prime[j]=1;}}}int main(){int n;int i,j;init();while(scanf("%d",&n)!=-1&&n){int ans=0;for(i=0;i<cnt&&IsPrime[i]<n;i++){for(j=i;j<cnt&&IsPrime[j]<n;j++){if(IsPrime[i]+IsPrime[j]==n)ans++;}}printf("%d\n",ans);}return 0;}