Codeforces 543A Writing Code

http://codeforces.com/problemset/problem/543/A

题目大意：n个人，一共要写m行程序，每个程序员每行出现的bug数为ai，要求整个程序出现的bug数不超过b的方案数.

思路:f[i][j]代表第m行，j个bug的方案数，n^3转移

#include<cstdio>
#include<cmath>
#include<iostream>
#include<algorithm>
#include<cstring>
int n,m,b,Mod;
int a[200005],f[505][2005];
int read(){
    int t=0,f=1;char ch=getchar();
    while (ch<'0'||ch>'9'){if (ch=='-') f=-1;ch=getchar();}
    while ('0'<=ch&&ch<='9') {t=t*10+ch-'0';ch=getchar();}
    return t*f;
}
int main(){
    n=read();m=read();b=read();Mod=read();
    for (int i=1;i<=n;i++) a[i]=read();
    f[0][0]=1;
    for (int i=1;i<=n;i++)
     for (int j=1;j<=m;j++)
      for (int k=b;k>=0;k--)
        if (k>=a[i])
            f[j][k]=(f[j][k]+f[j-1][k-a[i]])%Mod;
    int ans=0;
    for (int i=0;i<=b;i++)
     ans=(ans+f[m][i])%Mod;        
    printf("%d\n",ans);    
}