【poj3522】 Slim Span

// poj3522
#include<algorithm>
#include<iostream>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<cmath>
#define LL long long
#define inf 2147483640
#define Pi acos(-1.0)
#define free(a) freopen(a".in","r",stdin),freopen(a".out","w",stdout);
using namespace std;

const int maxn=5010;
struct edge {int u,v,w;}e[maxn];
int n,m,fa[maxn];

bool cmp(edge a,edge b) {
	return a.w<b.w;
}
int find(int x) {
	return fa[x]==x ? x : fa[x]=find(fa[x]);
}
int main() {
	while (scanf("%d%d",&n,&m)!=EOF && (n || m)) {
		for (int i=1;i<=m;i++) scanf("%d%d%d",&e[i].u,&e[i].v,&e[i].w);
		sort(e+1,e+1+m,cmp);
		int ans=inf;
		for (int l=1;l<=m;l++) {
			int cnt=0,flag=0;
			for (int i=1;i<=n;i++) fa[i]=i;
			for (int i=l;i<=m;i++) {
				int r1=find(e[i].u),r2=find(e[i].v);
				if (r1!=r2) cnt++,fa[r1]=r2;
				if (cnt==n-1) {flag=1;ans=min(ans,e[i].w-e[l].w);break;}
			}
			if (!flag) break;
		}
		if (ans==inf) printf("-1\n");
		else printf("%d\n",ans);
	}
	return 0;
}