水概率DP，ANS=sigma(min(a[i],a[i-1])/(a[i]*a[i-1]))
1 #include<bits/stdc++.h> 2 #define inc(i,l,r) for(int i=l;i<=r;i++) 3 #define dec(i,l,r) for(int i=l;i>=r;i--) 4 #define link(x) for(edge *j=h[x];j;j=j->next) 5 #define mem(a) memset(a,0,sizeof(a)) 6 #define inf 1e9 7 #define ll long long 8 #define succ(x) (1<<x) 9 #define lowbit(x) (x&(-x)) 10 #define NM 10000000+5 11 using namespace std; 12 int read(){ 13 int x=0,f=1;char ch=getchar(); 14 while(!isdigit(ch)){if(ch=='-')f=-1;ch=getchar();} 15 while(isdigit(ch))x=x*10+ch-'0',ch=getchar(); 16 return x*f; 17 } 18 int n,A,B,C,a[NM]; 19 double s; 20 int main(){ 21 freopen("data.in","r",stdin); 22 scanf("%d%d%d%d%d",&n,&A,&B,&C,a+1); 23 for (int i=2;i<=n;i++) a[i] = ((long long)a[i-1] * A + B) % 100000001; 24 for (int i=1;i<=n;i++) a[i] = a[i] % C + 1; 25 a[n+1]=a[1]; 26 inc(i,2,n+1)s+=(double)min(a[i],a[i-1])/a[i]/a[i-1]; 27 printf("%.3lf ",s); 28 return 0; 29 }