HDU1518 Square（DFS） 2016-07-24 15:08 49人阅读 评论(0) 收藏

Square

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

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#include <stdio.h>
#include <string.h>
#include <algorithm>
using namespace std;

int a[250], vis[205], sum, l, n;

int dfs(int x, int pos, int len)
{
    if (x == 3)
        return 1;
    int i;
    for (i = pos; i >= 0; i--)
    {
        if (!vis[i])
        {
            vis[i] = 1;
            if (len + a[i]<l)
            {
                if (dfs(x, i - 1, len + a[i]))
                    return 1;
            }
            else if (len + a[i] == l)
            {
                if (dfs(x + 1, n - 1, 0))
                    return 1;
            }
            vis[i] = 0;
        }
    }
    return 0;
}

int main()
{
    int t, i, j;
    scanf("%d", &t);
    while (t--)
    {
        sum = 0;
        scanf("%d", &n);
        for (i = 0; i<n; sum += a[i], i++)
            scanf("%d", &a[i]);
        l = sum / 4;
        memset(vis, 0, sizeof(vis));
        sort(a, a + n);
        if (l * 4 != sum || n<4 || l<a[n - 1])//剪枝
        {
            printf("no\n");
            continue;
        }
        if (dfs(0, n - 1, 0))
            printf("yes\n");
        else
            printf("no\n");
    }

    return 0;
}