UVA - 1395 Slim Span

#include<iostream>
#include<string>
#include<cstdio>
#include<set>
#include<map>
#include<stack>
#include<list>
#include<vector>
#include<queue>
#include<algorithm>
#include<cstring>
#include<cmath>
#include<fstream>

using namespace std;
typedef long long ll;

const int maxn = 10010,INF = 0x3f3f3f3f;
int m,n;

struct node{
int u,v,w;
bool operator < (const node &a) const{
return w < a.w;
}
};

vector<node> e;

int fa[maxn];

int find(int x){
if (x == fa[x]) return x;
else return fa[x] = find(fa[x]);
}

int kruskal(int st){
int sum = 1,k = st;
for(int i = 1;i <= n;++i) fa[i] = i;
for(;k < m;++k){
int x = find(e[k].u),y = find(e[k].v);
if (x != y) {
fa[x] = y;
sum++;
}
if (sum == n) return k;
}
return -1;
}

void build(){
int ans = INF;
int mark = -1;
for(int l = 0;l < m;++l){
if (e[l].w == mark) continue;
int r = kruskal(l);
if (r != -1){
mark = e[l].w;
ans = min(e[r].w - e[l].w,ans);
}
else break;
}
if (ans == INF) ans = -1;
cout << ans << endl;
}

void init(){
int u,v,w;
e.clear();
for(int i = 0;i < m;++i){
cin >> u >> v >> w;
e.push_back({u,v,w});
}
sort(e.begin(),e.end());
build();
}

int main(){
while(cin >> n >> m && n + m){
init();
}
}