素数类题目。POJ2689 POJ3978 POJ2262 POJ3001

POJ2689

#include<iostream>
using namespace std;
#define maxn 1047483640

bool prime[maxn];


int main()
{
	long long m,n;
	long long min_x,min_y,max_x,max_y;
	int max=-1;
	int min=1e9;
	int dis;
	long long  pre=0;
	bool cnt=true;
	cin>>m>>n;
	for(int i=2;i<n;i++)
	{
		if(i%2==0)prime[i]=false;
		else
		{
			prime[i]=true;
		}
	}
	prime[2]=true;
	for(int i=3;i*i<=n;i+=2)
	{
		if(prime[i])
		{
			for(int j=i+i;j<n;j+=i)
			{
				prime[j]=false;
			}
		}
	}
	for(int i=m;i<=n;i++)
	{
		if(prime[i]&&cnt)
		{
			pre=i;
			cnt=false;
			continue;
		}
		if(prime[i]&&!cnt)
		{
			dis=i-pre;
			if(dis>max)
			{
				max_x=pre;
				max_y=i;
				max=dis;
			}
			if(dis<min)
			{
				min_x=pre;
				min_y=i;
				min=dis;
			}
			pre=i;
		}
	}
	if(min==1e9)cout<<"There are no adjacent primes."<<endl;
	else
	{
		cout<<min_x<<","<<min_y<<" are closest, "<<max_x<<","<<max_y<<" are most distant."<<endl;
	}
	return 0;
}

POJ3978

#include<iostream>
using namespace std;
#define maxn 21474836

bool prime[maxn];


int main()
{
	int n,m;
	while(cin>>m>>n)
	{
		if(m==-1&&n==-1)break;
		for(int i=0;i<n;i++)
		{
			if(i%2==0)prime[i]=false;
			else
			{
				prime[i]=true;
			}
		}
		prime[2]=true;
		prime[1]=false;
		for(int i=3;i*i<=n;i+=2)
		{
			if(prime[i])
			{
				for(int j=i+i;j<n;j+=i)
				{
					prime[j]=false;
				}
			}
		}
		int cnt=0;
		for(int i=m;i<=n;i++)
		{
			if(prime[i])cnt++;
		}
		cout<<cnt<<endl;		
	}
	return 0;
}

POJ2262

#include<iostream>
using namespace std;
#define maxn 21474836

bool prime[maxn];


int main()
{
	int n=1000000;
	for(int i=2;i<n;i++)
	{
		if(i%2==0)prime[i]=false;
		else
		{
			prime[i]=true;
		}
	}
	prime[2]=true;
	for(int i=3;i*i<=n;i+=2)
	{
		if(prime[i])
		{
			for(int j=i+i;j<n;j+=i)
			{
				prime[j]=false;
			}
		}
	}
	int num;
	while(cin>>num&&num)
	{
		for(int i=2;i<=num;i++)
		{
			if(prime[i]&&prime[num-i])
			{
				cout<<num<<" = "<<i<<" + "<<num-i<<endl;
				break;
			}
		}		
	}
	return 0;
}

POJ3001

#include<iostream>
using namespace std;
#define maxn 21474836

bool prime[maxn];


int main()
{
	int n=10000000;
	for(int i=2;i<n;i++)
	{
		if(i%2==0)prime[i]=false;
		else
		{
			prime[i]=true;
		}
	}
	prime[2]=true;
	for(int i=3;i*i<=n;i+=2)
	{
		if(prime[i])
		{
			for(int j=i+i;j<n;j+=i)
			{
				prime[j]=false;
			}
		}
	}
	int a,d,m;
	while(cin>>a>>d>>m)
	{
		if(a==0&&d==0&&m==0)break;
		int cnt=0; 
		for(int i=a;i<1000000;i+=d)
		{
			if(prime[i])
			{
				cnt++;
				if(cnt==m)
				{
					cout<<i<<endl;
					break;
				}	
			}
		}		
	}


	return 0;
}