POJ 3552

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>

using namespace std;

const int VM=110;
const int INF=999999999;

struct Edge{
    int u,v;
    int cap;
}edge[VM*VM];

int n,m,ans,father[VM];

void makeSet(){
    for(int i=1;i<=n;i++){
        father[i]=i;
    }
}

int findSet(int x){
    if(x!=father[x]){
        father[x]=findSet(father[x]);
    }
    return father[x];
}

int cmp(Edge a,Edge b){
    return a.cap<b.cap;
}

void Kruskal(){
    sort(edge,edge+m,cmp);
    for(int i=0;i<m;i++){
        makeSet();
        int cnt=0,tmp=INF;
        for(int j=i;j<m;j++){
            int fx=findSet(edge[j].u);
            int fy=findSet(edge[j].v);
            if(fx!=fy){
                father[fy]=fx;
                cnt++;
                if(cnt==n-1){
                    tmp=edge[j].cap-edge[i].cap;
                    break;
                }
            }
        }
        if(tmp<ans)
            ans=tmp;
    }
}

int main(){

    //freopen("input.txt","r",stdin);

    while(~scanf("%d%d",&n,&m)){
        if(n==0 && m==0)
            break;
        for(int i=0;i<m;i++)
            scanf("%d%d%d",&edge[i].u,&edge[i].v,&edge[i].cap);
        ans=INF;
        Kruskal();
        if(ans!=INF)
            printf("%d\n",ans);
        else
            printf("-1\n");
    }
    return 0;
}