686 - Goldbach's conjecture (II)/686.cpp

   1 using namespace std;
   2 #include <algorithm>
   3 #include <iostream>
   4 #include <iterator>
   5 #include <sstream>
   6 #include <fstream>
   7 #include <cassert>
   8 #include <climits>
   9 #include <cstdlib>
  10 #include <cstring>
  11 #include <string>
  12 #include <cstdio>
  13 #include <vector>
  14 #include <cmath>
  15 #include <queue>
  16 #include <deque>
  17 #include <stack>
  18 #include <list>
  19 #include <map>
  20 #include <set>
  21
  22 #define foreach(x, v) for (typeof (v).begin() x = (v).begin(); x != (v).end(); ++x)
  23 #define For(i, a, b) for (int i=(a); i<(b); ++i)
  24 #define D(x) cout << #x " is " << x << endl
  25
  26 const int MAXN =  1 << 15 + 5;
  27 bool prime[MAXN];
  28
  29 int main(){
  30   memset(prime, true, sizeof prime);
  31   prime[0] = prime[1] = false;
  32   for (int i=4; i<MAXN; i += 2) prime[i] = false;
  33   for (int i=3; i<MAXN; i += 2){
  34     if (prime[i]){
  35       for (int j=i+i; j<MAXN; j += i) prime[j] = false;
  36     }
  37   }
  38
  39   int n;
  40   while (scanf("%d", &n)==1 && n){
  41     int m = n / 2, ans = prime[n-2];
  42     for (int a = 3; a <= m; a += 2){
  43       ans  += (prime[a] && prime[n-a]);
  44     }
  45     printf("%d\n", ans);
  46   }
  47   return 0;
  48 }