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;
}