ZOJ 1909 I-Square

https://vjudge.net/contest/67836#problem/I

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

代码：

#include <bits/stdc++.h>
using namespace std;

int n, ave, maxx, sum;
int a[50];
bool flag = false;
int vis[50];

void dfs(int num, int len, int start) {
    if(flag)
        return;

    if(num == 4) {
        flag = true;
        return ;
    }

    if(len == ave) {
        dfs(num + 1, 0, 1);
        if(flag)
            return ;
    }

    for(int i = start; i <= n; i ++) {
        if(vis[i] == 0 && len + a[i] <= ave) {
            vis[i] = 1;
            dfs(num, len + a[i], i + 1);
            vis[i] = 0;
            if(flag)
                return ;
        }
    }
}

int main() {
    int T;
    scanf("%d", &T);
    while(T --) {
        scanf("%d", &n);
        maxx = 0, sum = 0;
        for(int i = 1; i <= n; i ++) {
            scanf("%d", &a[i]);
            sum += a[i];
        }
        sort(a + 1, a + 1 + n);
        maxx = a[n];
        ave = sum / 4;
        if(sum % 4 != 0 || maxx > ave) {
            printf("no\n");
            continue;
        }
        memset(vis, 0, sizeof(vis));
        flag = false;
        dfs(0, 0, 1);
        if(flag)
            printf("yes\n");
        else
            printf("no\n");
    }
    return 0;
}

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