Gym 100463A Crossing Number（树状数组）

#include<bits/stdc++.h>
using namespace std;
#define LL long long
const int maxn = 1000000+500;
int c[maxn];
int lowbit(int x){return x&(-x);}
void update(int i,int d)
{
	while(i<=maxn)
	{
		c[i]+=d;
		i+=lowbit(i);
	}
}
int query(int i)
{
	int ans = 0;
	while(i)
	{
		ans+=c[i];
		i-=lowbit(i);
	}
	return ans;
}
int main()
{
	int cas=1;
	LL n,a,b;
	while(scanf("%lld%lld%lld",&n,&a,&b)!=EOF)
	{
		memset(c,0,sizeof(c));
		if(n==0 &&a==0&&b==0)
			break;
		LL ans = 0;
        for(int i = 0;i<n;i++)
		{
            LL num = (a*i+b)%n+1;
			ans+=query(num-1);
			update(num,1);
		}
        printf("Case %d: %lld\n",cas++,n*(n-1)/2-ans);
	}
}