CodeForces #309 Div.2 C. Kyoya and Colored Balls

C. Kyoya and Colored Balls

题目链接:http://codeforces.com/problemset/problem/554/C

题意:题目给出k种颜色的球,每个求各有Ci个,且要求第i种球其后紧跟第i+1种球.
求一共有多少种排法.

题解:
//由于题目要求第i种球其后紧跟第i+1种球.
//意味着必须确定每种颜色最后一个球的方法
//所以对于最后一种颜色,假设这个颜色有x个,总球数为y.
//那么最后一种颜色的球,必须有一个球,要放在最后一个位置,那么剩下的球就有C(x-1,y-1)种方法.
//依次从后往前类推,将每种情况相乘即可.

#include<stdio.h>#include<string.h>#include<string.h>#include<algorithm>using namespace std;#define ll __int64#define maxn 1000000007int a[1600];ll c[1050][1060];ll sum;int main() {    int n,m,sum1;    for(int i = 1; i <= 1000; i++)        c[i][0] = 1;    for(int i = 1; i <= 1000; i++) {        for(int j = 1; j <= i; j++) {            if(i == j)                c[i][j] = 1;            else if(i > j)                c[i][j] = (c[i - 1][j] + c[i - 1][j - 1]) % maxn;        }    }    while(~scanf("%d",&n)) {        for(int i = 1; i <= n; i++) {            scanf("%d",&a[i]);        }        sum1 = a[1];        sum = 1;        for(int i = 2; i <= n; i++) {            sum1 += a[i];            sum = (sum * c[sum1 - 1][a[i] - 1]) % maxn;        }        printf("%I64d\n",sum);      }    return 0;}