【数学】CSU 1810 Reverse (2016湖南省第十二届大学生计算机程序设计竞赛)

//
//by coolxxx
//#include<bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<string>
#include<iomanip>
#include<map>
#include<stack>
#include<queue>
#include<set>
#include<bitset>
#include<memory.h>
#include<time.h>
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
//#include<stdbool.h>
#include<math.h>
#define min(a,b) ((a)<(b)?(a):(b))
#define max(a,b) ((a)>(b)?(a):(b))
#define abs(a) ((a)>0?(a):(-(a)))
#define lowbit(a) (a&(-a))
#define sqr(a) ((a)*(a))
#define swap(a,b) ((a)^=(b),(b)^=(a),(a)^=(b))
#define mem(a,b) memset(a,b,sizeof(a))
#define eps (1e-8)
#define J 10000
#define mod 1000000007
#define MAX 0x7f7f7f7f
#define PI 3.14159265358979323
#define N 100004
using namespace std;
typedef long long LL;
int cas,cass;
int n,m,lll,ans;
LL aans;
LL e
,sum
,l
,r
;
LL a;
char s
;
int main()
{
#ifndef ONLINE_JUDGE
//    freopen("1.txt","r",stdin);
//    freopen("2.txt","w",stdout);
#endif
int i,j,k;
LL x,y;
//    for(scanf("%d",&cass);cass;cass--)
//    for(scanf("%d",&cas),cass=1;cass<=cas;cass++)
//    while(~scanf("%s",s))
while(~scanf("%d",&n))
{
aans=0;
scanf("%s",s);
n=strlen(s);
e
=1;e[n+1]=0;
for(i=n-1;i;i--)e[i]=(e[i+1]*10)%mod;
r
=1;r[n+1]=0;
for(i=n-1;i+i>=n;i--)r[i]=(r[i+1]+e[i]*(n-i+1))%mod;
l[1]=e[1];l[0]=0;
for(i=2;i+i<=n+2;i++)l[i]=(l[i-1]+e[i]*i)%mod;
sum[1]=e[1];
for(i=2;i<=n;i++)sum[i]=sum[i-1]+e[i];

for(i=1;i<=n;i++)
{
a=s[i-1]-'0';
if(a==0)continue;
x=i;y=n-i+1;
if(x>y)swap(x,y);
aans=(aans+a*l[min(x,y)-1])%mod;
aans=(aans+a*r[max(x,y)+1])%mod;
aans=(aans+(a*x*(sum[y]-sum[x-1]))%mod)%mod;
aans=(aans+(e[i]*a)%mod*(x*(x-1)/2+y*(y-1)/2)%mod)%mod;
}
printf("%lld\n",aans);
}
return 0;
}
/*
//

//
*/