题解 P2383 【狗哥玩木棒】

licone · 2016-08-10 22:47:02 · 题解

dfs+剪枝~

剪枝方法：随输入更新tot和max值，如果(tot%4)!=0或者maxx>(tot/4)直接输出no.

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
#define max(u,v) u>v ? u:v
int t,n,a[21],tot,maxx,k;
bool b[21];
bool dfs(int u,int v)
{
    if(u==tot && v==3) return 1;
    if(u==tot) v++,u=0;
    for(int i=1;i<=n;i++)
      if(!b[i] && (a[i]+u)<=tot)
      {
          b[i]=1;if(dfs(a[i]+u,v)) return 1;b[i]=0;
      }
    return 0;
}
int main()
{
    scanf("%d",&t);
    while(t--)
    {
        memset(b,0,sizeof(b));
        tot=0;k=1;maxx=0;
        scanf("%d",&n);
        for(int i=1;i<=n;i++) scanf("%d",&a[i]),tot+=a[i],maxx=max(maxx,a[i]);
        if((tot%4)!=0 || maxx>(tot/4)) printf("no\n"),k=0;tot/=4;
        sort(a+1,a+n+1);
        if(k)
        {
            if(dfs(0,0)) printf("yes\n");
            else printf("no\n");
        }
    }
    return 0;
}