Codeforces_327_C

http://codeforces.com/problemset/problem/327/C

等比求和相加，有mod的出现，所以要算逆元。

#include<iostream>
#include<cstring>
#include<cstdio>
#include<algorithm>
#define LL long long
#define MOD 1000000007
using namespace std;

char s[];
int k;

LL p_mod(LL a,LL b,LL c)
{
    LL ans = ;
    a = a%c;
    while(b > )
    {
        if(b% == )    ans = (ans*a)%c;
        b /= ;
        a = (a*a)%c;
    }
    return ans;
}

LL get_inv(LL a,LL c)
{
    return p_mod(a,c-,c);
}

int main()
{
    scanf("%s%d",s,&k);
    LL ans = ;
    int len = strlen(s);
    LL x = p_mod(,len,MOD);
    LL y = get_inv(x-,MOD);
    x = p_mod(x,k,MOD)-;
    LL z = ;
    for(int i = ;i < len;i++)
    {
        if(s[i] == '' || s[i] == '')
        {
            z = (z+p_mod(,i,MOD))%MOD;
        }
    }
    ans = (((z*x)%MOD*y)%MOD)%MOD;
    printf("%I64d\n",ans);
    return ;
}