POJ-2909-Goldbach's Conjecture
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这个题要求求出有多少对素数的和等于n
思路:筛选后直接枚举即可,注意枚举到中间值就行,不要重复计算素数对
代码:
#include<cstdio>#include<cstring>const int maxn=32800;bool ispri[maxn];int pri[maxn],cnt;void Init(){ ispri[1]=1; cnt=0; for(int i=2;i<maxn;i++) {if(!ispri[i]) pri[cnt++]=i;for(int j=2;i*j<maxn;j++) ispri[i*j]=1; }}int main(){ Init(); int n; while(scanf("%d",&n)&&n) {int m=n>>1,ans=0;for(int i=0;pri[i]<=m;i++) if(!ispri[n-pri[i]])ans++;printf("%d\n",ans); } return 0;}
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