嘛，一直以来蒟蒻都没怎么打过CF……现在还是蓝名狗……今天跟着zyf打了一场virtual，果断一题滚粗
Kyoya and Colored Balls
签到题，从前往后考虑，第$i$种球的最后一个一定要放在当前序列的最后一个位置，剩下的$a[i]-1$个可以在前面随便放……所以$ans=\prod_{i=2}^n \binom{s[i]-1}{a[i]-1} $
1 //Codeforces #309 A 2 #include<cstdio> 3 #include<cstring> 4 #include<cstdlib> 5 #include<iostream> 6 #include<algorithm> 7 #define rep(i,n) for(int i=0;i<n;++i) 8 #define F(i,j,n) for(int i=j;i<=n;++i) 9 #define D(i,j,n) for(int i=j;i>=n;--i) 10 #define pb push_back 11 using namespace std; 12 typedef long long LL; 13 inline int getint(){ 14 int r=1,v=0; char ch=getchar(); 15 for(;!isdigit(ch);ch=getchar()) if (ch=='-') r=-1; 16 for(; isdigit(ch);ch=getchar()) v=v*10-'0'+ch; 17 return r*v; 18 } 19 const int N=100010,P=1000000007; 20 /*******************template********************/ 21 LL n,a[N],s[N]; 22 LL sum,ans=1,fac[N],inv[N]; 23 LL Pow(LL a,int b){ 24 LL r=1; 25 for(;b;b>>=1,a=a*a%P) if (b&1) r=r*a%P; 26 return r; 27 } 28 LL C(int n,int m){ return fac[n]*inv[m]%P*inv[n-m]%P;} 29 int main(){ 30 #ifndef ONLINE_JUDGE 31 freopen("A.in","r",stdin); 32 freopen("A.out","w",stdout); 33 #endif 34 n=getint(); 35 fac[0]=fac[1]=1; 36 F(i,1,1000) fac[i]=fac[i-1]*i%P; 37 inv[1000]=Pow(fac[1000],P-2); inv[0]=1; 38 D(i,999,1) inv[i]=inv[i+1]*(i+1)%P; 39 40 F(i,1,n) a[i]=getint(),s[i]=(s[i-1]+a[i])%P; 41 ans=1; 42 F(i,2,n) ans=ans*C(s[i]-1,a[i]-1)%P; 43 cout <<ans<<endl; 44 return 0; 45 }