SGU231-Prime Sum 还是素数 - c++编程基础

231. Prime Sum

time limit per test: 0.5 sec.
memory limit per test: 4096 KB input: standard
output: standard

Find all pairs of prime numbers (A, B) such that A<=B and their sum is also a prime number and does not exceed N.
Input The input of the problem consists of the only integer N (1<=N<=10^6).
Output On the first line of the output file write the number of pairs meeting the requirements. Then output all pairs one per line (two primes separated by a space).
Sample test(s)
Input

4 Output

0 题目说的是，求两个素数的和还为素数的对数，并把符合条件的写下来。

因为大于2的素数都为奇数，所以两奇数和为偶数一定不是素数。剩下的就只有2和其他素数相加。

这样就转换为有多少个差为2的素数。

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                using namespace std; bool vis[1000007]; void isprime(int n) { int k=sqrt(n+0.5); for(int i=2; i<=k; i++) { if(!vis[i]) for(int j=i*i; j<=n; j+=i) vis[j]=1; } } int main() { int n; while(scanf(%d,&n)!=EOF) { memset(vis,0,sizeof(vis)); vis[0]=1,vis[1]=1; int count=0; isprime(n); for(int i=2; i<=n-2; i++) { if(!vis[i]&&!vis[i+2]&&(i+2)<=n) count++; } printf(%d ,count); if(count) { for(int i=2; i<=n-2; i++) { if(!vis[i]&&!vis[i+2]&&(i+2)<=n) printf(2 %d ,i); } } } return 0; }