HDU 1518 经典DFS

从大到小排序可以减少耗时，

从小到大会增加耗时;

注意回溯  和 剪枝

/*
 *HDU 1518
 *fuqiang11
 *DFS
 *2013/7/28
*/

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
#define maxn 20+3

int a[maxn];
bool visit[maxn];
int N,M,eva;
int cmp(int x, int y)
{
    return x > y;
}
int  DFS(int st,int sum, int num)
{
    if(num == 4)
    {
        return 1;
    }
    for(int i = st; i <= M; i++)
    {
        if(!visit[i])
        {
            if(sum + a[i] == eva)
            {
                visit[i] = true;
                if(DFS(1,0,num+1))  //搜索到一条边，继续搜（从起点开始搜索）
                    return 1;
                visit[i] = false;
            }
            else if(sum + a[i] < eva)
            {
                visit[i] = true;
                if(DFS(i+1,sum+a[i],num))
                    return 1;
                visit[i] = false;
            }
        }
    }
    return 0;
}
int main()
{
#ifndef ONLINE_JUDGE
    freopen("in","r",stdin);
#endif
    cin>>N;
    while(N--)
    {
        cin>>M;
        int sum = 0,MAX = 0;
        for(int i = 1; i <= M; i++)
        {
            cin>>a[i];
            sum += a[i];
//            MAX = max(MAX,a[i]);
        }
        eva = sum / 4;
        if(sum % 4 != 0 || M < 4)
        {
            puts("no");
            continue;
        }
        memset(visit,false,sizeof(visit));
        sort(a+1,a+M+1,cmp);
        if(!DFS(1,0,0))
            puts("no");
        else
            puts("yes");
    }
}