状压地爬做法 比较费空间，但是易懂

#include<bits/stdc++.h>
#define ll long long
using namespace std;
int n,w,a[20];
int f[20][1<<19],tot;
int main()
{
    scanf("%d%d",&n,&w);
    for(int i=1;i<=n;i++)
    {
        scanf("%d",&a[i]);
        f[i][1<<(i-1)]=a[i];
    }

    memset(f,0x3f,sizeof(f));
    f[1][0]=0;
    tot=(1<<n)-1;
    for(int i=1;i<=n;i++)
    {
        for(int j=0;j<=tot;j++)
        {
            if(f[i][j]!=0x3f3f3f3f)
            {
                for(int k=1;k<=n;k++)
                {
                    if(w-f[i][j]>=a[k]&&!(j&1<<(k-1)))
                    {
                        f[i][j|1<<(k-1)]=min(f[i][j|1<<(k-1)],f[i][j]+a[k]);
                    }
                    else f[i+1][j|1<<(k-1)]=min(f[i+1][j|1<<(k-1)],a[k]);
                }
            }
        }
        if(f[i][tot]!=0x3f3f3f3f)
        {
            printf("%d",i);
            return 0;
        }       
    }
}