题目链接：https://www.spoj.com/problems/HIGH/en/

       题意是有n个城市，要修m条高速公路，使这n个城市连通，问共有多少种方案。

       这道题就是求无向图的最小生成树的个数，所以直接用生成树计数(不取模的模板就好了)...不知道为啥现在贴的呆码显示出来的缩进都是乱的，凑合着看吧...

AC代码：

#include <bits/stdc++.h>
#define ll long long
#define maxn 55
#define inf 0x3f3f3f3f
using namespace std;
ll A[maxn][maxn];
ll B[maxn][maxn];
int n, m, k;

ll det(){
  ll res = 1;
	for (int i=1;i<=n;i++){
	  for (int j=i+1;j<=n;j++){
		while (B[j][i]){
		  ll t = B[i][i] / B[j][i];
		  for (int k=i;k<=n;k++) B[i][k] -= B[j][k] * t;
		  for (int k=i;k<=n;k++) swap(B[i][k], B[j][k]);
		  res = -res;
		}
	  }
	  if (B[i][i] == 0) return 0;
	  res = res * B[i][i];
	}
  if(res < 0) res = -res;
  return res;
}

int main()
{
  int T;
  scanf("%d",&T);
  while(T--){
    scanf("%d%d",&n,&m);
    memset(A, 0, sizeof(A));
    memset(B, 0, sizeof(B));
    for(int i=0;i<m;i++){
      int u, v;
      scanf("%d%d",&u,&v);
      A[u][v] = A[v][u] = 1;
    }
    for(int i=1;i<=n;i++){
      for(int j=i+1;j<=n;j++){
        if(A[i][j] == 1){
          B[i][i] ++;
          B[i][j] = -1;
		  B[j][j] ++;
		  B[j][i] = -1;
        }
      }
    }
    n --;
    ll ans = det();
    printf("%lld\n", ans);
  }
  return 0;
}