UVALIVE 3887 Slim Span

#include <map>
#include <set>
#include <list>
#include <cmath>
#include <ctime>
#include <deque>
#include <stack>
#include <queue>
#include <cctype>
#include <cstdio>
#include <string>
#include <vector>
#include <climits>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <algorithm>
#define LL long long
#define PI 3.1415926535897932626
using namespace std;
int gcd(int a, int b) {return a % b == 0 ? b : gcd(b, a % b);}
#define MAXN 110
struct node
{
        int u,v,w;
        friend bool operator < (const node &a,const node &b)
        {
                return a.w < b.w;
        }
}edge[MAXN * MAXN];
int N,M;
int p[MAXN];
int Find(int x) {return x == p[x] ? x : p[x] = Find(p[x]);}
int kruskal(int cur)
{
        int ans = -1,cnt = 0;
        for (int i = 0 ; i <= N; i++) p[i] = i;
        for (int i = 0 ; i < M; i++)
        {
                if (edge[i].w < cur) continue;
                int x = Find(edge[i].u), y = Find(edge[i].v);
                if (x != y)
                {
                        p[x] = y;
                        cnt++;
                        ans = max(ans,edge[i].w);
                }
                if (cnt == N - 1) break;
        }
        if (cnt == N - 1) return ans - cur;
        else return -1;
}
int main()
{
       // freopen("sample.txt","r",stdin);
        while (scanf("%d%d",&N,&M) != EOF)
        {
                if (N == 0 && M == 0) break;
                for (int i = 0 ; i < M; i++)
                        scanf("%d%d%d",&edge[i].u,&edge[i].v,&edge[i].w);
                int ans = INT_MAX;
                sort(edge,edge + M);
                for (int i = 0 ; i < M; i++)
                {
                        int tmp = kruskal(edge[i].w);
                        if (tmp == -1) break;
                        else ans = min (ans,tmp);
                }
                if (ans == INT_MAX) puts("-1");
                else printf("%d\n",ans);
        }
        return 0;
}