problem_10_3_1.py

INF = float('inf')

def tpath():
    global weights
    edges.sort(key = lambda x: x[2])
    weights = {edges[i][2] for i in range(m)}
    weights = sorted(list(weights))
    ret = INF
    for i in range(len(weights)):
        min_upperbound = min_upperbound_kruskal(i)
        ret = min(ret, min_upperbound - weights[i])
    return ret

def find(a):
    if a == parent[a]:
        return a
    return find(parent[a])

def union(a,b):
    if a == -1 or b == -1:
        return max(a,b)
    p, q = find(a), find(b)
    if p != q:
        if rank[p] > rank[q]:
            p, q = q, p
        parent[p] = q
        if rank[p] == rank[q]:
            rank[q] += 1
        
    return q



def min_upperbound_kruskal(low):
    global parent, rank
    parent = list(range(n))
    rank = [1] * n
    for i in range(m):
        a, b, weight = edges[i]
        if weight >= weights[low]:
            union(a, b)
            if find(0) == find(n-1):
                return weight
    return INF

def min_diff():
    ret = INF
    for i in range(len(weights)):
        min_upperbound = min_upperbound(i)
        ret = min(ret, min_upperbound - weights[i])
    return ret


    
for _ in range(int(input())):
    n, m = map(int, input().split())
    edges = [list(map(int, input().split())) for _ in range(m)]
    ret = tpath()
    print(0 if ret == INF else ret)