题目：
http://www.lydsy.com/JudgeOnline/problem.php?id=1050
将边从小到大排序，枚举边权最小的边k，从第k条边开始一条一条边插入，用并查集维护，直到s、t连通。
#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <algorithm>
using namespace std;
const int MaxN=501;
const int MaxM=5001;
const double oo=100000001;
struct line
{
int x,y;
int v;
};
int n,m,s,t,ansa,ansb;
double ans=oo;
int father[MaxN];
line a[MaxM];
void init()
{
cin>>n>>m;
for (int i=0;i<m;i++)
{
cin>>a[i].x>>a[i].y>>a[i].v;
}
cin>>s>>t;
}
bool cmp(line a,line b)
{
return a.v<b.v;
}
int find(int x)
{
if (father[x]==x)
{
return x;
}
return father[x]=find(father[x]);
}
void work(int st)
{
for (int i=1;i<=n;i++)
{
father[i]=i;
}
for (int i=st;i<m;i++)
{
int x=find(a[i].x),y=find(a[i].y);
if (x!=y)
{
father[x]=y;
}
if (find(s)==find(t))
{
if ((double)a[i].v/(double)a[st].v<ans)
{
ans=(double)a[i].v/(double)a[st].v;
ansa=a[i].v;
ansb=a[st].v;
}
return;
}
}
}
int gcd(int a,int b)
{
if (b==0)
{
return a;
}
return gcd(b,a%b);
}
int main()
{
init();
sort(a,a+m,cmp);
for (int i=0;i<m;i++)
{
work(i);
}
if (ans==oo)
{
cout<<"IMPOSSIBLE"<<endl;
}
else
{
int k=gcd(ansa,ansb);
cout<<ansa/k;
if (ansb/k!=1)
{
cout<<'/'<<ansb/k<<endl;
}
else
{
cout<<endl;
}
}
return 0;
}