DP offset array

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A way to learn, can deal with the transfer to the knapsack problem negative

Code is very easy

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
using namespace std;
int n,a[403],b[403],f[800003],ans,sum;
inline int rd()
{
    int x=0,p=1;
    char a=getchar();
    while((a<48||a>57)&&a!='-')
        a=getchar();
    if(a=='-')
        p=-p,a=getchar();
    while(a>47&&a<58)
        x=(x<<1)+(x<<3)+(a&15),a=getchar();
    return x*p;
}
int main()
{
    n=rd();
    for(int i=1; i<=n; i++)
        a[i]=rd(),b[i]=rd();
    for(int i=1; i<=n; i++)
        if(a[i]>0)
            sum+=a[i];
    sum*=2;
    for(int i=0; i<=sum; i++)
        f[i]=-1e9;
    f[sum/2]=0;
    for(int i=1; i<=n; i++)
    {
        if(a[i]>=0)
            for(int j=sum; j>=a[i]; j--)
                f[j]=max(f[j],f[j-a[i]]+b[i]);
        else
            for(int j=0; j<=sum+a[i]; j++)
                f[j]=max(f[j],f[j-a[i]]+b[i]);
    }
    for(int i=sum/2; i<=sum; i++)
        if(f[i]>0)
            ans=max(ans,f[i]+i-sum/2);
    cout<<ans;
    return 0;
}