【Codeforces 258E】 Devu and Flowers

【题目链接】

http://codeforces.com/contest/451/problem/E

【算法】

容斥原理

【代码】

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int P = 1e9 + ;

int i,j,n,s,ans,MASK;
ll m,t;
ll a[],f[];
int inv[];

inline int power(int a,int n)
{
        int res = ,b = a;
        while (n)
        {
                if (n & ) res = 1ll * res * b % P;
                b = 1ll * b * b % P;
                n >>= ;
        }
        return res;
}
inline int C(ll x,int y)
{
        int i,res = ;
        if (y <  || x < y || x < ) return ;
        if (x ==  || y == ) return ;
        x %= P;
        for (i = x; i >= x - y + ; i--) res = 1ll * res * i % P;
        for (i = ; i <= y; i++) res = 1ll * res * inv[i] % P;
        return res;
}

int main()
{

        scanf("%d%I64d",&n,&m);
        for (i = ; i <= n; i++) scanf("%I64d",&a[i]);
        for (i = ; i <= ; i++) inv[i] = power(i,P-);
        MASK =  << n;
        ans = ;
        for (i = ; i < MASK; i++)
        {
                if (i == ) ans = (ans + C(n+m-,n-)) % P;
                else
                {
                        s = ;
                        t = n + m;
                        for (j = ; j < n; j++)
                        {
                                if (i & ( << j))
                                {
                                        s++;
                                        t -= a[j+];
                                }
                        }
                        t -= (s + );
                        if (s & ) ans = (ans - C(t,n-) + P) % P;
                        else ans = (ans + C(t,n-)) % P;
                }
         }
        printf("%d\n",ans);

        return ;

}