最大边和最小边之差最小的生成树 UVA 1394

//看看会不会爆int!数组会不会少了一维！
//取物问题一定要小心先手胜利的条件
#include <bits/stdc++.h>
using namespace std;
#define LL long long
#define ALL(a) a.begin(), a.end()
#define pb push_back
#define mk make_pair
#define fi first
#define se second
const int inf = 0x3f3f3f3f;
const int maxn = 100 + 5;
struct Edge{
    int u, v, val;
    Edge(int u = 0, int v = 0, int val = 0): u(u), v(v), val(val){}
    bool operator < (const Edge &a) const{
        return val < a.val;
    }
}edges[maxn * maxn];
int par[maxn];
int n, m;

void init(){for (int i = 1; i <= n; i++) par[i] = i;}
int pfind(int x){
    if (par[x] == x) return x;
    return par[x] = pfind(par[x]);
}

bool solve(int l, int r){
    for (int i = l; i <= r; i++){
        int pu = pfind(edges[i].u);
        int pv = pfind(edges[i].v);
        if (pu == pv) continue;
        par[pu] = pv;
    }
    int p = pfind(1);
    for (int i = 2; i <= n; i++){
        if (p != pfind(i)) return false;
    }
    return true;
}

int main(){
    while (scanf("%d%d", &n, &m) && n){
        for (int i = 1; i <= m; i++) {
            int u, v, val; scanf("%d%d%d", &u, &v, &val);
            edges[i] = Edge(u, v, val);
        }
        int ans = inf;
        sort(edges + 1, edges + m + 1);
        for (int l = 1; l <= m; l++){
            init();
            for (int r = l; r <= m; r++){
                if (solve(l, r)){
                    ans = min(ans, edges[r].val - edges[l].val);
                    break;
                }
            }
        }
        if (ans == inf) ans = -1;
        printf("%d\n", ans);
    }
    return 0;
}