一个很city的问题

https://www.bnuoj.com/v3/contest_show.php?cid=7775#problem/E

a[i]=(a[i-1]<<1)%mod+i-1

#include<stdio.h>#define mod 10111009int main(){    int m,n,i,j;    int a[1000+10]={0};    for(i=1;i<=1000;i++)        a[i]=(a[i-1]<<1)%mod+i-1;    while(scanf("%d",&n)==1)    printf("%d\n",a[n]);}