PKU 2262 Goldbach's Conjecture

上星期遗下的题目还有一道没有完成

PKU 2262 Goldbach's Conjecture

http://acm.pku.edu.cn/JudgeOnline/problem?id=2262

主要是用高效点的求素数的方法

 

#include <iostream>

using namespace std;

const int N = 1000005;

bool a[N];

int p[N];

// bool hash[1000005]; 记录某数是否为素数

void Prime2(int n) //线性法,参数表示要打多少内的素数

{

   memset(a, 0, n*sizeof(a[0]));

// memset(hash,0,sizeof(hash));

   int num = 0, i, j;

   for(i = 2; i < n; ++i)

   {

       if(!(a[i]))

            {

                      p[num++] = i;

//                 hash[i] = 1;

            }

       for(j = 0; (j<num && i*p[j]<n); ++j)

            {

           a[i*p[j]] = 1;

           if(!(i%p[j])) break;

       }

   }

}

int main()

{

         int i,j,k,n,m,t;

         Prime2(1000001);

         while (scanf("%d",&n)!=EOF && n)

         {

                   for (i=0;p[i]<=n/2;i++)

                   {

                            if (hash[n - p[i]])

                            {

                                     printf("%d = %d + %d/n",n,p[i],n-p[i]);

                                     break;

                            }

                   }

         }

         return 0;

}