Buggy python submission (verdict: Runtime Error):

import sys

MEMO = dict()

def search(n, data):
    global MEMO

    if n in MEMO:
        return MEMO[n]
    else:
        if not n in data:
            MEMO[n] = 0
            return 0

        MEMO[n] = max(search(i, data) for i in data[n]) + 1
        return MEMO[n]


def main():
    sys.setrecursionlimit(65535)
    N, M = map(int, input().split())
    data = dict()
    for _ in range(M):
        x, y = map(int, input().split())
        if x not in data:
            data[x] = [y]
        else:
            data[x].append(y)

    print(max(search(n, data) for n in range(1, N+1)))



if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/python3
import sys

MEMO = dict()

def search(n, data):
    global MEMO

    if n in MEMO:
        return MEMO[n]
    else:
        if not n in data:
            MEMO[n] = 0
            return 0

        MEMO[n] = max(search(i, data) for i in data[n]) + 1
        return MEMO[n]


def main():
    # sys.setrecursionlimit(65535)
    sys.setrecursionlimit(10**9)
    N, M = map(int, input().split())
    data = dict()
    for _ in range(M):
        x, y = map(int, input().split())
        if x not in data:
            data[x] = [y]
        else:
            data[x].append(y)

    print(max(search(n, data) for n in range(1, N+1)))



if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#!/snap/bin/pypy3
import sys

MEMO = dict()

def search(n, data):
    global MEMO

    if n in MEMO:
        return MEMO[n]
    else:
        if not n in data:
            MEMO[n] = 0
            return 0

        MEMO[n] = max(search(i, data) for i in data[n]) + 1
        return MEMO[n]


def main():
    sys.setrecursionlimit(65535)
    N, M = map(int, input().split())
    data = dict()
    for _ in range(M):
        x, y = map(int, input().split())
        if x not in data:
            data[x] = [y]
        else:
            data[x].append(y)

    print(max(search(n, data) for n in range(1, N+1)))



if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/python3
import sys

MEMO = dict()

def search(n, data):
    global MEMO

    if n in MEMO:
        return MEMO[n]
    else:
        if not n in data:
            MEMO[n] = 0
            return 0

        MEMO[n] = max(search(i, data) for i in data[n]) + 1
        return MEMO[n]


def main():
    # sys.setrecursionlimit(65535)
    sys.setrecursionlimit(10**9)
    N, M = map(int, input().split())
    data = dict()
    for _ in range(M):
        x, y = map(int, input().split())
        if x not in data:
            data[x] = [y]
        else:
            data[x].append(y)

    print(max(search(n, data) for n in range(1, N+1)))



if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#!/snap/bin/pypy3

MEMO = dict()

def search(n, data):
    global MEMO

    if n in MEMO:
        return MEMO[n]
    else:
        if not n in data:
            MEMO[n] = 0
            return 0

        MEMO[n] = max(search(i, data) for i in data[n]) + 1
        return MEMO[n]


def main():
    N, M = map(int, input().split())
    data = dict()
    for _ in range(M):
        x, y = map(int, input().split())
        if x not in data:
            data[x] = [y]
        else:
            data[x].append(y)

    print(max(search(n, data) for n in range(1, N+1)))



if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/python3
import sys

MEMO = dict()

def search(n, data):
    global MEMO

    if n in MEMO:
        return MEMO[n]
    else:
        if not n in data:
            MEMO[n] = 0
            return 0

        MEMO[n] = max(search(i, data) for i in data[n]) + 1
        return MEMO[n]


def main():
    # sys.setrecursionlimit(65535)
    sys.setrecursionlimit(10**9)
    N, M = map(int, input().split())
    data = dict()
    for _ in range(M):
        x, y = map(int, input().split())
        if x not in data:
            data[x] = [y]
        else:
            data[x].append(y)

    print(max(search(n, data) for n in range(1, N+1)))



if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#!/snap/bin/pypy3

MEMO = dict()

def search(n, data):
    global MEMO
    
    if n in MEMO:
        return MEMO[n]
    else:
        startfrom_n = [element[1] for element in data if element[0] == n]
        if not startfrom_n:
            MEMO[n] = 0
            return 0

        MEMO[n] = max(search(i, data) for i in startfrom_n) + 1
        return MEMO[n]


def main():
    N, M = map(int, input().split())
    data = [tuple(map(int, input().split())) for _ in range(M)]


    print(max(search(n, data) for n in range(1, N+1)))



if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/python3
import sys

MEMO = dict()

def search(n, data):
    global MEMO

    if n in MEMO:
        return MEMO[n]
    else:
        if not n in data:
            MEMO[n] = 0
            return 0

        MEMO[n] = max(search(i, data) for i in data[n]) + 1
        return MEMO[n]


def main():
    # sys.setrecursionlimit(65535)
    sys.setrecursionlimit(10**9)
    N, M = map(int, input().split())
    data = dict()
    for _ in range(M):
        x, y = map(int, input().split())
        if x not in data:
            data[x] = [y]
        else:
            data[x].append(y)

    print(max(search(n, data) for n in range(1, N+1)))



if __name__ == '__main__':
    main()