题解见：http://blog.csdn.net/libin56842/article/details/46650209
注意这里的组合数取模~~~
1 /*Author :usedrose */ 2 /*Created Time :2015/8/7 13:31:44*/ 3 /*File Name :2.cpp*/ 4 #pragma comment(linker, "/STACK:1024000000,1024000000") 5 #include <cstdio> 6 #include <iostream> 7 #include <algorithm> 8 #include <sstream> 9 #include <cstdlib> 10 #include <cstring> 11 #include <climits> 12 #include <vector> 13 #include <string> 14 #include <ctime> 15 #include <cmath> 16 #include <deque> 17 #include <queue> 18 #include <stack> 19 #include <set> 20 #include <map> 21 #define INF 0x3f3f3f3f 22 #define eps 1e-8 23 #define pi acos(-1.0) 24 #define MAXN 1110 25 #define MAXM 1000110 26 #define OK cout << "ok" << endl; 27 #define o(a) cout << #a << " = " << a << endl 28 #define o1(a,b) cout << #a << " = " << a << " " << #b << " = " << b << endl 29 using namespace std; 30 typedef long long LL; 31 32 LL n; 33 LL a[MAXN]; 34 LL fac[MAXM]; 35 const LL mod = 1e9+7; 36 37 LL quick_pow(LL a, LL b) 38 { 39 LL c = 1; 40 while (b) { 41 if (b&1) c = c*a%mod; 42 b >>= 1; 43 a = a*a%mod; 44 } 45 return c%mod; 46 } 47 48 LL calc(LL m,LL i) 49 { 50 return ((fac[m]%mod)*(quick_pow((fac[i]*fac[m-i])%mod, mod-2)%mod))%mod; 51 } 52 53 int main() 54 { 55 //freopen("data.in","r",stdin); 56 //freopen("data.out","w",stdout); 57 cin.tie(0); 58 ios::sync_with_stdio(false); 59 fac[0] = 1; 60 for (int i = 1;i < MAXM; ++ i) 61 fac[i] = (fac[i-1]*i)%mod; 62 LL ans = 1, sum = 0; 63 cin >> n; 64 for (int i = 1;i <= n; ++ i) { 65 cin >> a[i]; 66 sum += a[i]; 67 } 68 for (int i = n;i >= 1;-- i) { 69 ans *= calc(sum-1, a[i]-1); 70 ans %= mod; 71 sum -= a[i]; 72 } 73 cout << ans << endl; 74 return 0; 75 }