重要节点

70% 通过，不知道卡在哪里啦，很奇怪，哪位大神能解答一下？

import queue
def zhongyao_dian(n,m,lists):
    Number_count = 0
    S = [0 for _ in range(n)]
    T = [0 for _ in range(n)]
    matrix = [[0 for _ in range(n)]for _ in range(n)]
    graph = []
    for i in range(n):
        graph.append([])
    stack = queue.Queue(maxsize=n)

    for i in lists:
        if i[1] not in graph[i[0]-1]:
            graph[i[0]-1].append(i[1]-1)
    for i in range(n):
        stack.put(i)
        while not stack.empty():
            cur = stack.get()
            for j in graph[cur]:
                if matrix[i][j] == 0:
                    matrix[i][j] = 1
                    stack.put(j)
    for i in range(len(matrix)):
        for j in range(len(matrix[i])):
            if matrix[i][j] == 1:
                S[i] += 1
                T[j] += 1
    for i in range(n):
        if T[i] - S[i] >0:
            Number_count += 1
    return Number_count
if __name__ == "__main__":
    n, m = [int(i) for i in input().split()]
    lists = []
    for i in range(m):
        lists.append([int(i) for i in input().split()])
    quchong_list = []
    for i in lists:
        if i not in quchong_list:
            quchong_list.append(i)
    print(zhongyao_dian(n, len(quchong_list), quchong_list))