1 #include<cstdio> 2 #include<algorithm> 3 #include<cstring> 4 using namespace std; 5 int n,m,fa[101],rank[101]; 6 void clear(){for(int i=1;i<=n;i++) fa[i]=i; memset(rank,0,sizeof(rank));} 7 int findroot(int x)  8 { 9     if(fa[x]==x) return x;10     int rt=findroot(fa[x]);11     fa[x]=rt;12     return rt;13 }14 void Union(int U,int V)15 {16     if(rank[U]<rank[V]) fa[U]=V;17     else18       {19         fa[V]=U;20         if(rank[U]==rank[V]) rank[U]++;21       }22 }23 struct Edge{int u,v,w;};24 bool cmp(const Edge &a,const Edge &b){return a.w<b.w;}25 Edge edges[5001];26 int tot,ans,maxv;27 int main()28 {29     while(1)30       {31           scanf("%d%d",&n,&m);32           if(!n) break;33           for(int i=1;i<=m;i++) scanf("%d%d%d",&edges[i].u,&edges[i].v,&edges[i].w);34           sort(edges+1,edges+m+1,cmp); ans=2147483647;35           for(int j=1;j<=m;j++)36             {37                 tot=0; clear();38                 for(int i=j;i<=m;i++)39                 {40                     int f1=findroot(edges[i].u),f2=findroot(edges[i].v);41                     if(f1!=f2) {Union(f1,f2); tot++; if(tot==n-1) {maxv=edges[i].w; goto WIN;}}42                 }43               continue;44               WIN:ans=min(ans,maxv-edges[j].w);45             }46           printf("%d\n",ans!=2147483647 ? ans : -1);47       }48     return 0;49 }