Problem:
Problem F: Slim Span Given an undirected weighted graph G , you should find one of spanning trees specified as follows. The graph G is an ordered pair ( V , E ), where V is a set of vertices { v 1 , v 2 , ... , v n } and E is a set of undirected edges { e 1 , e 2 , ... , e m }. Each edge e ∈ E has its weight w ( e ). A spanning tree T is a tree (a connected subgraph without cycles) which connects all the n
vertices with n - 1 edges. The slimness of a spanning tree T is defined as the difference between
the largest weight and the smallest weight among the n - 1 edges of T .   Figure 5: A graph G and the weights of the edges  For example, a graph G in Figure 5(a) has four vertices { v 1 , v 2 , v 3 , v 4 } and five undirected edges { e 1 , e 2 , e 3 , e 4 , e 5 }. The weights of the edges are w ( e 1 ) = 3, w ( e 2 ) = 5, w ( e 3 ) = 6, w ( e 4 ) = 6, w ( e 5 ) = 7 as shown in Figure 5(b).   Figure 6: Examples of the spanning trees of G   There are several spanning trees for G . Four of them are depicted in Figure 6(a)-(d). The spanning tree T a in Figure 6(a) has three edges whose weights are 3, 6 and 7. The largest weight
is 7 and the smallest weight is 3 so that the slimness of the tree T a is 4. The slimnesses of
spanning trees T b , T c and T d shown in Figure 6(b), (c) and (d) are 3, 2 and 1, respectively. You
can easily see the slimness of any other spanning tree is greater than or equal to 1, thus the
spanning tree T d in Figure 6(d) is one of the slimmest spanning trees whose slimness is 1. Your job is to write a program that computes the smallest slimness. Input The input consists of multiple datasets, followed by a line containing two zeros separated by a space. Each dataset has the following format.  n     m  a 1  b 1  w 1 .
  .
  . a m  b m  w m  Every input item in a dataset is a non-negative integer. Items in a line are separated by a space.  n is the number of the vertices and m the number of the edges. You can assume 2 ≤ n ≤ 100 and 0 ≤ m ≤ n ( n - 1)/2. a k and b k ( k = 1, ... , m ) are positive integers less than or equal to n , which represent the two vertices v a k and v b k connected by the k th edge e k . w k is a positive integer less than or equal to 10000, which indicates the weight of e k . You can assume that the graph G = ( V , E ) is simple, that is, there are no self-loops (that connect the same vertex) nor parallel edges (that are two or more edges whose both ends are the same two vertices). Output For each dataset, if the graph has spanning trees, the smallest slimness among them should be
printed. Otherwise, -1 should be printed. An output should not contain extra characters. Sample Input 4 5
1 2 3
1 3 5
1 4 6
2 4 6
3 4 7
4 6
1 2 10
1 3 100
1 4 90
2 3 20
2 4 80
3 4 40
2 1
1 2 1
3 0
3 1
1 2 1
3 3
1 2 2
2 3 5
1 3 6
5 10
1 2 110
1 3 120
1 4 130
1 5 120
2 3 110
2 4 120
2 5 130
3 4 120
3 5 110
4 5 120
5 10
1 2 9384
1 3 887
1 4 2778
1 5 6916
2 3 7794
2 4 8336
2 5 5387
3 4 493
3 5 6650
4 5 1422
5 8
1 2 1
2 3 100
3 4 100
4 5 100
1 5 50
2 5 50
3 5 50
4 1 150
0 0 Output for the Sample Input 1
20
0
-1
-1
1
0
1686
50

Solution (Python):
import math,string,itertools,fractions,heapq,collections,re,array,bisect,sys,random,time,copy,functools

sys.setrecursionlimit(10**7)
inf = 10**20
eps = 1.0 / 10**10
mod = 10**9+7
dd = [(-1,0),(0,1),(1,0),(0,-1)]
ddn = [(-1,0),(-1,1),(0,1),(1,1),(1,0),(1,-1),(0,-1),(-1,-1)]

def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x)-1 for x in sys.stdin.readline().split()]
def LF(): return [float(x) for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def I(): return int(sys.stdin.readline())
def F(): return float(sys.stdin.readline())
def S(): return input()
def pf(s): return print(s, flush=True)

class UnionFind:
    def __init__(self, size):
        self.table = [-1 for _ in range(size)]

    def find(self, x):
        if self.table[x] < 0:
            return x
        else:
            self.table[x] = self.find(self.table[x])
            return self.table[x]

    def union(self, x, y):
        s1 = self.find(x)
        s2 = self.find(y)
        if s1 != s2:
            if self.table[s1] <= self.table[s2]:
                self.table[s1] += self.table[s2]
                self.table[s2] = s1
            else:
                self.table[s2] += self.table[s1]
                self.table[s1] = s2
            return True
        return False

    def subsetall(self):
        a = []
        for i in range(len(self.table)):
            if self.table[i] < 0:
                a.append((i, -self.table[i]))
        return a

def main():
    rr = []

    while True:
        n,m = LI()
        if n == 0:
            break

        a = sorted([LI()[::-1] for _ in range(m)])
        for i in range(2,n+1):
            a.append([inf, 1, i])

        r = inf
        for i in range(m):
            uf = UnionFind(n+1)
            mt = 0
            uc = 0
            for t,x,y in a[i:]:
                if uf.union(x,y):
                    mt = t
                    uc += 1
                    if uc == n-1:
                        break
            if mt == inf:
                break
            tr = mt - a[i][0]
            if r > tr:
                r = tr

        if r == inf:
            r = -1
        rr.append(r)


    return '\n'.join(map(str, rr))



print(main())