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| #include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <algorithm>
#include <sstream>
#include <map>
#include <set>
#define REP(i,k,n) for(int i=k;i<n;i++)
#define rep(i,n) for(int i=0;i<n;i++)
#define INF 1<<30
#define pb push_back
#define mp make_pair
using namespace std;
typedef long long ll;
typedef pair<int,int> P;
struct UnionFind {
vector<int> par,rank;
int N;
UnionFind(int n) {
N = n;
par.resize(n);
rank.resize(n);
rep(i,n) {
par[i] = i;
rank[i] = 0;
}
}
int find(int x) {
if(par[x] == x) return x;
else return par[x] = find(par[x]);
}
void unite(int x,int y) {
x = find(x);
y = find(y);
if(x == y) return;
if(rank[x] < rank[y]) {
par[x] = y;
}
else {
par[y] = x;
if(rank[x] == rank[y]) rank[x]++;
}
}
bool same(int x,int y) {
return find(x) == find(y);
}
int size() {
int cnt = 0;
rep(i,N) if(find(i) == i) cnt++;
return cnt;
}
};
int main() {
int n, m;
while(cin >> n >> m) {
if(n == 0 && m == 0) break;
vector<int> a(m), b(m), c(m);
map<int, vector<P> > es;
rep(i, m) {
cin >> a[i] >> b[i] >> c[i];
a[i]--; b[i]--;
es[c[i]].push_back(P(a[i], b[i]));
}
int ans = INF;
REP(i, 1, 10001) {
if(es[i].size() == 0) continue;
UnionFind uf(n);
REP(j, i, 10001) {
if(es[j].size() == 0) continue;
rep(k, es[j].size()) {
int a = es[j][k].first;
int b = es[j][k].second;
if(uf.same(a, b)) continue;
uf.unite(a, b);
}
if(uf.size() == 1) {
ans = min(ans, j - i);
break;
}
}
}
if(ans == INF) cout << -1 << endl;
else cout << ans << endl;
}
return 0;
}
|