#include <map>#include <set>#include <list>#include <cmath>#include <ctime>#include <deque>#include <stack>#include <queue>#include <cctype>#include <cstdio>#include <string>#include <vector>#include <climits>#include <cstdlib>#include <cstring>#include <iostream>#include <algorithm>#define LL long long#define PI 3.1415926535897932626using namespace std;int gcd(int a, int b) {return a % b == 0 ? b : gcd(b, a % b);}#define MAXN 110struct node{ int u,v,w; friend bool operator < (const node &a,const node &b) { return a.w < b.w; }}edge[MAXN * MAXN];int N,M;int p[MAXN];int Find(int x) {return x == p[x] ? x : p[x] = Find(p[x]);}int kruskal(int cur){ int ans = -1,cnt = 0; for (int i = 0 ; i <= N; i++) p[i] = i; for (int i = 0 ; i < M; i++) { if (edge[i].w < cur) continue; int x = Find(edge[i].u), y = Find(edge[i].v); if (x != y) { p[x] = y; cnt++; ans = max(ans,edge[i].w); } if (cnt == N - 1) break; } if (cnt == N - 1) return ans - cur; else return -1;}int main(){ // freopen("sample.txt","r",stdin); while (scanf("%d%d",&N,&M) != EOF) { 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].w); int ans = INT_MAX; sort(edge,edge + M); for (int i = 0 ; i < M; i++) { int tmp = kruskal(edge[i].w); if (tmp == -1) break; else ans = min (ans,tmp); } if (ans == INT_MAX) puts("-1"); else printf("%d\n",ans); } return 0;}
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