SP11469 SUBSET - Balanced Cow Subsets

题目链接：

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define re register
#define pb push_back
#define fi first
#define se second
const int N=2e6+10;
void read(int &a)
{
    a=0;int d=1;char ch;
    while(ch=getchar(),ch>'9'||ch<'0')
        if(ch=='-')
            d=-1;
    a=ch^48;
    while(ch=getchar(),ch>='0'&&ch<='9')
        a=(a<<3)+(a<<1)+(ch^48);
    a*=d;
}
int n,m;
int a[30],cnt,f[N];
map <int,int> vis;
vector <int> v[N];
void dfs1(int i,int sum,int now)
{
    if(i>=m)
    {
        if(!vis.count(sum)) vis[sum]=++cnt;
        v[vis[sum]].pb(now);
        return;
    }
    dfs1(i+1,sum,now);
    dfs1(i+1,sum+a[i],now|(1<<i));
    dfs1(i+1,sum-a[i],now|(1<<i));
}
void dfs2(int i,int sum,int now)
{
    if(i>=n)
    {
        if(vis.count(sum))
        {
            int x=vis[sum];
            for(auto j:v[x]) f[j|now]=1;
        }
        return;
    }
    dfs2(i+1,sum,now);
    dfs2(i+1,sum+a[i],now|(1<<i));
    dfs2(i+1,sum-a[i],now|(1<<i));
}
int main()
{
    read(n);m=n>>1;
    for(re int i=0;i<n;i++) read(a[i]);
    dfs1(0,0,0),dfs2(m,0,0);
    int ans=0;
    for(re int i=1;i<(1<<n);i++) ans+=f[i];
    printf("%d",ans);
    return 0;
}