Lightoj1259——Goldbach`s Conjecture（哥德巴赫猜想）

Description

Goldbach’s conjecture is one of the oldest unsolved problems in number theory and in all of mathematics. It states:

Every even integer, greater than 2, can be expressed as the sum of two primes [1].

Now your task is to check whether this conjecture holds for integers up to 107.

Input

Input starts with an integer T (≤ 300), denoting the number of test cases.

Each case starts with a line containing an integer n (4 ≤ n ≤ 107, n is even).

Output

For each case, print the case number and the number of ways you can express n as sum of two primes. To be more specific, we want to find the number of (a, b) where

1) Both a and b are prime

2) a + b = n

3) a ≤ b

Sample Input

2

6

4

Sample Output

Case 1: 1

Case 2: 1

自己的模板总是差那么多内存，换了一个就过了。。。

#include <iostream>
#include <cstring>
#include <string>
#include <vector>
#include <queue>
#include <cstdio>
#include <map>
#include <cmath>
#include <algorithm>
#define INF 0x3f3f3f3f
#define MAXN 10000005
#define mod 1000000007
using namespace std;
bool notprime[MAXN];
int primes[700005]; //素数从i=1开始
void get_prime()
{
notprime[1]=true;
for(int i=2;i<MAXN;++i)
if(!notprime[i])
{
primes[++primes[0]]=i;
for(long long j=(long long)i*i;j<MAXN;j+=i)
notprime[j]=true;
}
}
int main()
{
get_prime();
int t,cnt=1;
scanf("%d",&t);
while(t--)
{
int n,ans=0;
scanf("%d",&n);
for(int i=1; primes[i]<=n/2; ++i)
if(!notprime[n-primes[i]])
ans++;
printf("Case %d: %d\n",cnt++,ans);
}
return 0;
}