uva 1395 slim span MST

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<queue>
#include<string>
#include<cstring>
#include<iomanip>
#include<iostream>
#include<stack>
#include<cmath>
#include<map>
#include<vector>
#define ll long long
#define inf 0x3f3f3f3f
#define INF 1e9
#define bug1 cout<<"bug1"<<endl;
#define bug2 cout<<"bug2"<<endl;
#define bug3 cout<<"bug3"<<endl;
using namespace std;
#define sf scanf
#define pf printf
#define mem(a,b) memset(a,b,sizeof(a));
int n,m;
const int maxn=105;
struct Edge{
int u,v,c;
bool friend operator<(Edge a,Edge b){
return a.c<b.c;
}
}edge[maxn*maxn/2];
int fa[maxn];
int find(int x){
return x==fa[x]?x:fa[x]=find(fa[x]);
}
int main(){
while(~scanf("%d%d",&n,&m)&&n+m){
for(int i=1;i<=m;++i){
int u,v,c;
sf("%d%d%d",&edge[i].u,&edge[i].v,&edge[i].c);
}
sort(edge+1,edge+1+m);
int L=1,R=L+n-1;
int ans=INF;
for(L=1;L<=m-n+2;++L){//这个地方我因为一开始写的是m-n+1,结果样例答案都错了。。
//我应该花一些时间来看下第一个样例。然后觉得m-n+1 还是多少。。
for(R=L;R<=m;++R){//这个地方也是，我想用R=L+n-1，还是会错
for(int k=1;k<=n;++k)fa[k]=k;
int fg=0;
int cnt=0;
for(int i=L;i<=R;++i){
int fx=find(edge[i].u);int fy=find(edge[i].v);
if(fx!=fy){
fa[fx]=fy;
cnt++;
}
if(cnt==n-1){
ans=min(ans,edge[R].c-edge[L].c);
fg=1;
break;
}
}
if(fg)break;
}
}
if(ans<INF)pf("%d\n",ans);
else puts("-1");
}

}