hdu 1518 Square（深搜+剪枝）

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=1518

题目大意:根据题目所给的几条边，来判断是否能构成正方形，一个很好的深搜应用，注意剪枝，以防超时！

#include <iostream>
#include <cstdio>
#include<algorithm>
#include <cstring>
using namespace std;
int ap[30],visit[30];
int l,n;
int dfs(int len,int gen,int iqq)
{
    if (gen==3)
        return 1;
    for(int i=iqq; i<n; i++)
    {
        //cout<<visit[len]<<endl;
        if (!visit[i])
        {
            visit[i]=1;
            if (len+ap[i]==l)
            {
                //cout<<len<<endl;
                if(dfs(0,gen+1,0))
                    return 1;
            }
            else if (len+ap[i]<l)
            {
                //cout<<len<<endl;
                if(dfs(len+ap[i],gen,i)) return 1;
            }
            visit[i]=0;
        }
    }
    return 0;
}
int main ()
{
    int t,sum;
    while (cin>>t)
    {
        while (t--)
        {
            cin>>n;
            sum=0;
            //Max=0;
            for (int i=0; i<n; i++)
            {
                cin>>ap[i];
                sum+=ap[i];
            }
            memset(visit,0,sizeof(visit));
            sort(ap,ap+n);
            if (sum%4==0&&n>=4&&ap[n-1]<=sum/4)
            {

                l=sum/4;
                if (dfs(0,0,0))
                    printf ("yes\n");
                else
                    printf ("no\n");
                //cout<<n<<endl;
            }
            else printf ("no\n");
        }
    }
}