HDU 1518 Square 搜索

Problem Description

Given a set of sticks of various lengths, is it possible to join them end-to-end to form a square?

Input

The first line of input contains N, the number of test cases. Each test case begins with an integer 4 <= M <= 20, the number of sticks. M integers follow; each gives the length of a stick - an integer between 1 and 10,000.

Output

For each case, output a line containing "yes" if is is possible to form a square; otherwise output "no".

Sample Input

3
4 1 1 1 1
5 10 20 30 40 50
8 1 7 2 6 4 4 3 5

Sample Output

yes
no
yes

 题意 ：给几组数据 看每组数据 能否所实用完 并组成一个正方形

代码：

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
int q[22];
int vis[22];
int cmp(int a,int b)
{
    return a<b;
}
int t,n,s;
void dfs(int num,int k,int length)            //num 是已完毕的边 k是 数组的位置  length是当前边的长度
{
    if(t==1)
        return ;
    if(num==4)
    {
        t=1;
        return  ;
    }
    if(length==s)
    {
        dfs(num+1,0,0);
        if(t==1)
            return ;
    }
    for(int i=k; i<n; i++)
    {
        if(length+q[i]<=s&&vis[i]==0)
        {
            vis[i]=1;
            dfs(num,i+1,length+q[i]);
            vis[i]=0;
        }
    }
}
int main()
{
    int m;
    scanf("%d",&m);
    while(m--)
    {
        t=0;
        s=0;
        scanf("%d",&n);
        for(int i=0; i<n; i++)
        {
            scanf("%d",&q[i]);
            s=s+q[i];
        }
        if(s%4!=0)
        {
            printf("no\n");
            continue;
        }
        sort(q,q+n,cmp);
        s=s/4;
        if(q[n-1]>s)
        {
            printf("no\n");
            continue;
        }
        dfs(0,0,0);
        if(t==1)
            printf("yes\n");
        else
            printf("no\n");

    }
    return 0;
}

标准的回溯  可惜 我没有看到这个问题 要运行数据

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