选数 ssl 2669 优美的暴力

题目大意

给出n个数a[i]，现在可以在其中任意选出若干个数，问有多少种选择方案，使得这几个数可以分成两个和相等的集合。

分析

点一下dalao

code

#include<iostream>#include<cstring>#include<cstdio>#include<cmath>#include<queue>#include<algorithm>using namespace std;struct arr{    int a,aa;}a[70000],b[70000];int f[100];int ff[20000000];int num_a;int num_b;int num,n;int ans=0;bool cmp(arr a,arr b){    return a.a<b.a;}int dfs1(int x,int num,int m){    if (x==(n/2+1))    {        if (m==0) return 0;        a[++num_a].a=num;        a[num_a].aa=m;        return 0;    }    for (int i=-1;i<=1;i++)    {        if ((i==1)||(i==-1))            dfs1(x+1,num+f[x]*i,m*2+1);        if (i==0)            dfs1(x+1,num+f[x]*i,m*2);    }}int dfs2(int x,int num,int m){    if (x==(n+1))    {        if (m==0) return 0;        b[++num_b].a=num;        b[num_b].aa=m;        return 0;    }    for (int i=-1;i<=1;i++)    {        if ((i==1)||(i==-1))            dfs2(x+1,num+f[x]*i,m*2+1);        if (i==0)            dfs2(x+1,num+f[x]*i,m*2);    }}int main(){    //freopen("number.in", "r", stdin);    //freopen("number.out", "w", stdout);    scanf("%d",&n);    for (int i=1;i<=n;i++)    {        scanf("%d",&f[i]);    }    dfs1(1,0,0);    dfs2(n/2+1,0,0);    sort(a+1,a+num_a+1,cmp);    sort(b+1,b+num_b+1,cmp);    int link_a=1,link_b=num_b,link_bb=num_b;    for (link_a;link_a<=num_a;link_a++)    {        int k=0;        while ((a[link_a].a+b[link_b].a>0)&&(link_b>0))            link_b--;        link_bb=link_b;        while ((a[link_a].a+b[link_b].a==0)&&(link_b>0))        {            if (!ff[(a[link_a].aa<<(n/2+1))+b[link_b].aa])                ff[(a[link_a].aa<<(n/2+1))+b[link_b].aa]=1;            else k++;            link_b--;        }        ans=ans+(link_bb-link_b)-k;        link_b=link_bb;    }    for (int i=1;i<=num_a;i++)        if ((a[i].a==0)&&(ff[a[i].aa<<(n/2+1)]==0))             ans++,ff[a[i].aa<<(n/2+1)]=1;    for (int i=1;i<=num_b;i++)        if ((b[i].a==0)&&(ff[b[i].aa]==0))             ans++,ff[b[i].aa]=1;    printf("%d",ans);    fclose(stdin);    fclose(stdout);}