标签：图论
On an alien planet, every extraterrestrial is born with a number. If the sum of two numbers is a prime number, then two extraterrestrials can be friends. But every extraterrestrial can only has at most one friend. You are given all number of the extraterrestrials, please determining the maximum number of friend pair.
There are several test cases.
Each test start with positive integers N(1 ≤ N ≤ 100), which means there are N extraterrestrials on the alien planet.
The following N lines, each line contains a positive integer pi ( 2 ≤ pi ≤10^18),indicate the i-th extraterrestrial is born with pi number.
The input will finish with the end of file.
For each the case, your program will output maximum number of friend pair.
3
2
2
3
4
2
5
3
8
1
2
#include<stdio.h>
#include<math.h>
#include <algorithm>
#include<string.h>
#include <time.h>
using namespace std;
#define ll long long
//-----大素数的判断-------
ll mult_mod(ll a,ll b,ll n)
{
ll s=0;
while(b)
{
if(b&1) s=(s+a)%n;
a=(a+a)%n;
b>>=1;
}
return s;
}
ll pow_mod(ll a,ll b,ll n)
{
ll s=1;
while(b)
{
if(b&1) s=mult_mod(s,a,n);
a=mult_mod(a,a,n);
b>>=1;
}
return s;
}
int Prime(ll n)
{
ll u=n-1,pre,x;
int i,j,k=0;
if(n==2||n==3||n==5||n==7||n==11) return 1;
if(n==1||(!(n%2))||(!(n%3))||(!(n%5))||(!(n%7))||(!(n%11))) return 0;
for(;!(u&1);k++,u>>=1);
srand((ll)time(0));
for(i=0;i<5;i++)
{
x=rand()%(n-2)+2;
x=pow_mod(x,u,n);
pre=x;
for(j=0;j<k;j++)
{
x=mult_mod(x,x,n);
if(x==1&&pre!=1&&pre!=(n-1))
return 0;
pre=x;
}
if(x!=1) return 0;
}
return 1;
}
int n,ok[105][105],mark[105],vist[105];
int DFS(int x)
{
for(int i=1;i<=n;i++)
if(ok[x][i]&&vist[i]==0)
{
vist[i]=1;
if(mark[i]==0||DFS(mark[i]))
{
mark[i]=x; return 1;
}
}
return 0;
}
int main()
{
ll a[105];
while(scanf("%d",&n)>0)
{
for(int i=1;i<=n;i++)
{
scanf("%lld",&a[i]);
}
memset(ok,0,sizeof(ok));
for(int i=1;i<n;i++)
for(int j=i+1;j<=n;j++)
if(Prime(a[i]+a[j]))
ok[i][j]=ok[j][i]=1;
int ans=0;
memset(mark,0,sizeof(mark));
for(int i=1;i<=n;i++)
{
memset(vist,0,sizeof(vist));
ans+=DFS(i);
}
printf("%d\n",ans/2);
}
}
CSU1552: Friends（快速判断大数是不是素数+二分匹配）
标签：图论
原文地址：http://blog.csdn.net/u010372095/article/details/44872717
