Codeforces Round #258 (Div. 2) E . Devu and Flowers 母函数+公式

题解：http://codeforces.com/blog/entry/13181

#include<stdio.h>#include<string.h>#include<math.h>#include<iostream>#include<algorithm>using namespace std;#define N 55#define ll long long#define inf 0x3f3f3f3fll mod=1e9+7;ll f[22];ll n,s;ll inv(ll a)//求逆元{return a == 1 ? 1 : (long long)(mod - mod / a) * inv(mod % a) % mod;}ll choose(ll m,ll k)求组合数C(m,k){ll ret=1;for(ll i=0;i<k;i++)ret=ret*((m-i)%mod)%mod*inv(i+1)%mod;return ret;}ll go(ll i,ll s)//求解系数{if(s<0) return 0;if(i==n)return choose(n+s-1,n-1);return (go(i+1,s)-go(i+1,s-f[i])+mod)%mod;}int main(){cin>>n>>s;for(int i=0;i<n;i++){cin>>f[i];f[i]++;}cout<<(go(0,s)+mod)%mod<<endl;//system("pause");}