Coprime Integers (2018 Pacific Northwest Regional Contest)

题目链接: http://codeforces.com/group/NVaJtLaLjS/contest/242322
题意: 求x $\epsilon$(a,b)到与y $\epsilon$(c,d)的gcd(x,y)==1的个数
思路: 直接套莫比乌斯模板，注意容斥一下就行了

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn=1e7+5;
bool check[maxn];
int prime[maxn];
int mu[maxn];
void get_mo()
{
	mu[1]=1;
	int cnt=0;
	for(int i=2;i<maxn;i++)
	{
		if(!check[i])
		{
			prime[cnt++]=i;
			mu[i]=-1;
		}
		for(int j=0;j<cnt;j++)
		{
			if(i*prime[j]>maxn)
				break;
			check[i*prime[j]]=true;
			if(i%prime[j]==0){
				mu[i*prime[j]]=0;
				break;
			}
			else
			{
				mu[i*prime[j]]=-mu[i];
			}

		}
	}
	for(int i=1;i<maxn;i++)
	{
		mu[i]+=mu[i-1];
	}

}
ll sl(int x,int y)
{
	if(x>y)
		swap(x,y);
	ll ans=0;
	for(int i=1,last;i<=x;i=last+1)
	{
		last=min(x/(x/i),y/(y/i));
		ans+=1ll*(mu[last]-mu[i-1])*(x/i)*(y/i);
	}
	return ans;
}
int main()
{
	get_mo();
	int a,b,c,d;
	cin>>a>>b>>c>>d;
	ll ans=sl(b,d)-sl(a-1,d)-sl(c-1,b)+sl(a-1,c-1);
	cout<<ans<<endl;
	return 0;
}