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[100005];
int k;

LL p_mod(LL a,LL b,LL c)
{
    LL ans = 1;
    a = a%c;
    while(b > 0)
    {
        if(b%2 == 1)    ans = (ans*a)%c;
        b /= 2;
        a = (a*a)%c;
    }
    return ans;
}

LL get_inv(LL a,LL c)
{
    return p_mod(a,c-2,c);
}

int main()
{
    scanf("%s%d",s,&k);
    LL ans = 0;
    int len = strlen(s);
    LL x = p_mod(2,len,MOD);
    LL y = get_inv(x-1,MOD);
    x = p_mod(x,k,MOD)-1;
    LL z = 0;
    for(int i = 0;i < len;i++)
    {
        if(s[i] == ‘0‘ || s[i] == ‘5‘)
        {
            z = (z+p_mod(2,i,MOD))%MOD;
        }
    }
    ans = (((z*x)%MOD*y)%MOD)%MOD;
    printf("%I64d\n",ans);
    return 0;
}