【组合数学】 Codeforces Round #295 (Div. 1) C - Pluses everywhere

#include <iostream>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <bitset>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <climits>
#include <cstdlib>
#include <cmath>
#include <time.h>
#define maxn 100005
#define maxm 100005
#define eps 1e-7
#define mod 1000000007
#define INF 0x3f3f3f3f
#define PI (acos(-1.0))
#define lowbit(x) (x&(-x))
#define mp make_pair
#define ls o<<1
#define rs o<<1 | 1
#define lson o<<1, L, mid
#define rson o<<1 | 1, mid+1, R
#define pii pair<int, int>
#pragma comment(linker, "/STACK:16777216")
typedef long long LL;
typedef unsigned long long ULL;
//typedef int LL;
using namespace std;
LL qpow(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base;base=base*base;b/=2;}return res;}
LL powmod(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base%mod;base=base*base%mod;b/=2;}return res;}
// head

char s[maxn];
LL f[maxn];
LL g[maxn];
int n, m;

void read()
{
scanf("%d%d", &n, &m);
scanf("%s", s+1);
f[0] = 1;
for(int i = 1; i <= n; i++) f[i] = f[i-1] * i % mod;
g
= powmod(f
, mod - 2);
for(int i = n-1; i >= 0; i--) g[i] = g[i+1] * (i+1) % mod;
}

inline LL C(int k1, int k2)
{
if(k2 > k1) return 0;
return f[k1] * g[k2] % mod * g[k1 - k2] % mod;
}

void work()
{
LL ans = 0, sum = 0, mul = 1;
for(int i = n; i >= 1; i--) {
LL t = s[i] - '0';
ans = (ans + (sum + mul * C(i-1, m) % mod) * t) % mod;
sum = (sum + C(i-2, m-1) * mul) % mod;
mul = mul * 10 % mod;
}
printf("%I64d\n", ans);
}

int main()
{
read();
work();

return 0;
}