codeforce 189A 完全背包之必须装满问题

http://codeforces.com/problemset/problem/189/A

A. Cut Ribbon

time limit per test
2 seconds

memory limit per test
256 megabytes

input
standard input

output
standard output

Polycarpus has a ribbon, its length is n. He wants to cut the ribbon in a way that fulfils the following two conditions:

After the cutting each ribbon piece should have length a, b or c.

After the cutting the number of ribbon pieces should be maximum.

Help Polycarpus and find the number of ribbon pieces after the required cutting.

Input

The first line contains four space-separated integers n, a, b and c (1 ≤ n, a, b, c ≤ 4000) —
the length of the original ribbon and the acceptable lengths of the ribbon pieces after the cutting, correspondingly. The numbers a, b and c can
coincide.

Output

Print a single number — the maximum possible number of ribbon pieces. It is guaranteed that at least one correct ribbon cutting exists.

Sample test(s)

input

5 5 3 2

output

2

input

7 5 5 2

output

2

Note

In the first example Polycarpus can cut the ribbon in such way: the first piece has length 2, the second piece has length 3.

In the second example Polycarpus can cut the ribbon in such way: the first piece has length 5, the second piece has length 2.

这个是完全背包里面的必须装满的一类问题，注意和不必须装满的区别，代码上有区别~~

#include <iostream>

using namespace std;

const int INF=1e9;

int f[4444];
int n,m;
int a[4];

int main()
{
cin>>n>>a[0]>>a[1]>>a[2];
for (int i=1; i<=n; i++) f[i]=-INF;//区别一
f[0]=0;
for (int i=0; i<=2; i++)
{
for (int j=0; j<=n-a[i]; j++)
{
if (f[j]!=-INF)//区别二
{
f[j+a[i]]=max(f[j+a[i]],f[j]+1);
}
}
}
cout<<f
<<endl;
return 0;
}