n, m = map(int, input().split())

tot_edges, fa, ans, cnt = sorted([tuple(map(int, input().split())) for _ in range(m)], key=lambda x: x[-1]), list(range(n + 1)), 0, 0

def father(a):
    if fa[a] != a: fa[a] = father(fa[a])
    return fa[a]

for u, v, w in tot_edges:
    if father(u) != father(v):  fa[father(u)], ans, cnt = father(v), ans + w, cnt + 1

print(ans) if cnt == n - 1 else print('impossible')

上面写法不推荐，仅用于测试下python3的精简性，阅读性较高的代码如下，共15行：

n, m = map(int, input().split())
tot_edges = sorted([tuple(map(int, input().split())) for _ in range(m)], key=lambda x: x[-1])

fa = list(range(n + 1))

def father(a):
    if fa[a] == a: return a
    fa[a] = father(fa[a])
    return fa[a]

ans, cnt = 0, 0
for u, v, w in tot_edges:
    fu, fv = father(u), father(v)
    if fu == fv: continue
    fa[fu] = fv
    ans += w
    cnt += 1

print(ans) if cnt == n - 1 else print('impossible')