UVa 1395 Slim Span【最小生成樹】

先將邊權從小到大排序，然後對於邊權集合$[l,r]$，若此時的邊權集合足以構成一棵樹，那就記錄答案：$ans=min\{tree[r].z-tree[l].z\}$

#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#define ll long long
#define rep(i,x,y) for(ll i=(x);i<=(y);i++)
#define repl(i,x,y) for(ll i=(x);i<(y);i++)
#define repd(i,x,y) for(ll i=(x);i>=(y);i--)
using namespace std;

const ll N=1e4+5;
const ll Inf=1e18;

ll n,m,f[N];

struct node {
	ll x,y,z;
}edge[N];

inline ll read() {
    ll x=0;char ch=getchar();bool f=0;
    while(ch>'9'||ch<'0'){if(ch=='-')f=1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=(x<<1)+(x<<3)+ch-'0';ch=getchar();}
    return f?-x:x;
}

ll find(ll x) {
	return f[x]==x?x:f[x]=find(f[x]);
}

bool cmp(node p,node q) {
	return p.z<q.z;
}

int main() {
//	freopen("uva1395.txt","r",stdin);
	
	while(true) {
		n=read(),m=read();ll ans=Inf;
		
		if(!n) return 0;
		
		rep(i,1,m) edge[i].x=read(),edge[i].y=read(),edge[i].z=read();
		
		if(n==2&&m==1) {
			puts("0");continue;
		}
		
		sort(edge+1,edge+1+m,cmp);
		
		rep(i,1,m) {
			rep(j,1,n) f[j]=j;
			
			ll mi=0,mx=0,cnt=0;
			
			rep(j,i,m) {
				ll x=edge[j].x;
				ll y=edge[j].y;
				ll z=edge[j].z;
				
				ll fx=find(x),fy=find(y);
				
				if(fx!=fy) {
					cnt++;
					f[fy]=fx;
					if(cnt==1) mi=z;
					if(cnt==n-1) {
						mx=z;
						break;
					}
				}
			}
			
			if(mi&&mx) ans=min(ans,mx-mi);
		}
		
		if(ans==Inf) printf("-1\n");
		else printf("%lld\n",ans);
	}
	
	return 0;
}