HDU 1518 Square

dfs+剪枝。

AC代码例如以下：

///dfs+剪枝  62MS 300K

#include<iostream>
#include<cstring>
#include<algorithm>
#include<cstdio>
using namespace std;

int n,m;
int a[50];
int vis[50];

bool cmp(int a,int b)
{
return a>b;
}

int dfs(int v,int cur,int bg)
{
if(cur==4)
return 1;
int i;
for(i=bg;i<n;i++)
{
int flag=0;
if(!vis[i]&&a[i]+v==m)
{
flag=1;
vis[i]=1;
if(dfs(0,cur+1,0))
return 1;
vis[i]=0;
return 0;
}
if(!vis[i]&&a[i]+v<m)
{
flag=1;
vis[i]=1;
if(dfs(a[i]+v,cur,i+1))
return 1;
vis[i]=0;
if(v==0)
return 0;
}
while(flag&&i+1<n&&a[i+1]==a[i])
i++;
}
return 0;
}

int main()
{
int t;
int i;
scanf("%d",&t);
while(t--)
{
int sum=0;
scanf("%d",&n);
for(i=0;i<n;i++)
{
scanf("%d",&a[i]);
sum+=a[i];
}
sort(a,a+n,cmp);
if(sum%4!=0||a[0]>sum/4)
{
printf("no\n");
continue;
}
m=sum/4;
memset(vis,0,sizeof vis);
if(dfs(0,0,0))
printf("yes\n");
else printf("no\n");

}
return 0;
}