Problem p02985 (python)

Fixed submission:
# -*- coding: utf-8 -*-
import bisect
import heapq
import math
import random
import sys
from collections import Counter, defaultdict, deque
from decimal import ROUND_CEILING, ROUND_HALF_UP, Decimal
from functools import lru_cache, reduce
from itertools import combinations, combinations_with_replacement, product, permutations
from operator import add, mul, sub

sys.setrecursionlimit(100000)
input = sys.stdin.readline


def read_int():
    return int(input())


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input().strip()


def read_str_n():
    return list(map(str, input().split()))


def error_print(*args):
    print(*args, file=sys.stderr)


def mt(f):
    import time

    def wrap(*args, **kwargs):
        s = time.time()
        ret = f(*args, **kwargs)
        e = time.time()

        error_print(e - s, 'sec')
        return ret

    return wrap


class Mod:
    def __init__(self, m):
        self.m = m

    def add(self, a, b):
        return (a + b) % self.m

    def sub(self, a, b):
        return (a - b) % self.m

    def mul(self, a, b):
        return ((a % self.m) * (b % self.m)) % self.m

    def div(self, a, b):
        return self.mul(a, pow(b, self.m-2, self.m))

    def pow(self, a, b):
        return pow(a, b, self.m)


@mt
def slv(N, K, AB):
    M = Mod(10**9+7)
    def P(n, k):
        ret = 1
        for i in range(n, n-k, -1):
            ret = M.mul(ret, i)
        return ret
    
    g = defaultdict(list)
    for a, b in AB:
        g[a].append(b)
        g[b].append(a)
    
    q = [1]
    done = set()
    ans = K
    while q:
        u = q.pop()
        
        for v in g[u]:
            if v not in done:
                done.add(v)
                q.append(v)
        c = len(g[u])-1
        if u == 1:
            c += 1
        
        done.add(u)
        if u == 1:
            ans = M.mul(ans, P(K-1, c))
        else:
            ans = M.mul(ans, P(K-2, c))
    return ans


def main():
    N, K = read_int_n()
    AB = [read_int_n() for _ in range(N-1)]
    print(slv(N, K, AB))


if __name__ == '__main__':
    main()

Buggy submission (verdict: Runtime Error):
# -*- coding: utf-8 -*-
import bisect
import heapq
import math
import random
from collections import Counter, defaultdict, deque
from decimal import ROUND_CEILING, ROUND_HALF_UP, Decimal
from fractions import Fraction
from functools import lru_cache, reduce
from itertools import combinations, combinations_with_replacement, product, permutations, accumulate
from operator import add, mul, sub, itemgetter, attrgetter


import sys
# sys.setrecursionlimit(10**6)
# readline = sys.stdin.buffer.readline
readline = sys.stdin.readline

INF = 2**62-1


def read_int():
    return int(readline())


def read_int_n():
    return list(map(int, readline().split()))


def read_float():
    return float(readline())


def read_float_n():
    return list(map(float, readline().split()))


def read_str():
    return readline().strip()


def read_str_n():
    return readline().strip().split()


def ep(*args):
    print(*args, file=sys.stderr)


def mt(f):
    import time

    def wrap(*args, **kwargs):
        s = time.perf_counter()
        ret = f(*args, **kwargs)
        e = time.perf_counter()

        ep(e - s, 'sec')
        return ret

    return wrap


@mt
def slv(N, K, AB):
    g = defaultdict(list)
    for a, b in AB:
        g[a].append(b)
        g[b].append(a)

    def dfs(u, p, k1, k2):
        r = K - k1 - k2


        cu = 0
        for v in g[u]:
            if v == p:
                continue
            r *= dfs(v, u, 1, cu+k1)
            r %= (10**9+7)
            cu += 1

        return r

    return dfs(1, -1, 0, 0)



def main():
    N, K = read_int_n()
    AB = [read_int_n() for _ in range(N-1)]
    print(slv(N, K, AB))


if __name__ == '__main__':
    main()