Problem p02902 (python)

Fixed submission:
import sys
input = sys.stdin.readline
sys.setrecursionlimit(10**5)

N, M = map(int, input().split())
G = [[] for _ in range(N+1)]
for _ in range(M):
    A, B = map(int, input().split())
    G[A].append(B)

def dfs(v, o):
    for c in G[v]:
        if c == o:
            return [v]
    for c in G[v]:
        if visited[c]:
            continue
        visited[c] = True
        ret = dfs(c, o)
        if ret:
            return [v] + ret

for v in range(1, N+1):
    visited = [False] * (N+1)
    cur = dfs(v, v)
    if cur:
        break
else:
    print(-1)
    exit()

while True:
    cur_set = set(cur)
    unused = None
    for i in range(len(cur)):
        u, v = cur[i], cur[(i+1) % len(cur)]
        for c in G[u]:
            if c == v:
                continue
            elif c in cur_set:
                unused = (i, cur.index(c))
                break

    if not unused:
        break
    else:
        t, s = unused
        nxt = []
        i = s
        while i != t:
            nxt.append(cur[i])
            i = (i + 1) % len(cur)
        nxt.append(cur[t])
        cur = nxt

print(len(cur))
print(*cur, sep='\n')

Buggy submission (verdict: Runtime Error):
vimport sys
input = sys.stdin.readline
sys.setrecursionlimit(10**5)

N, M = map(int, input().split())
G = [[] for _ in range(N+1)]
for _ in range(M):
    A, B = map(int, input().split())
    G[A].append(B)

def dfs(v, o):
    for c in G[v]:
        if c == o:
            return [v]
    for c in G[v]:
        ret = dfs(c, o)
        if ret:
            return [v] + ret

for v in range(1, N+1):
    cur = dfs(v, v)
    if cur:
        break
else:
    print(-1)
    exit()

while True:
    cur_set = set(cur)
    unused = None
    for i in range(len(cur)):
        u, v = cur[i], cur[(i+1) % len(cur)]
        for c in G[u]:
            if c == v:
                continue
            elif c in cur_set:
                unused = (i, cur.index(c))
                break

    if not unused:
        break
    else:
        t, s = unused
        nxt = []
        i = s
        while i != t:
            nxt.append(cur[i])
            i = (i + 1) % len(cur)
        nxt.append(cur[t])
        cur = nxt
print(len(cur))
print(*cur, sep='\n')

Buggy submission (verdict: Runtime Error):
import sys
input = sys.stdin.readline

N, M = map(int, input().split())
G = [[] for _ in range(N+1)]
for _ in range(M):
    A, B = map(int, input().split())
    G[A].append(B)

def dfs(v, o):
    ret = None
    for c in G[v]:
        if c == o:
            return [v]
    else:
        for c in G[v]:
            ret = dfs(c, o)
            if ret:
                return [v] + ret

for v in range(1, N+1):
    cur = dfs(v, v)
    if cur:
        break
else:
    print(-1)
    exit()

while True:
    cur_set = set(cur)
    unused = None
    for i in range(len(cur)):
        u, v = cur[i], cur[(i+1) % len(cur)]
        for c in G[u]:
            if c == v:
                continue
            elif c in cur_set:
                unused = (i, cur.index(c))
                break

    if not unused:
        break
    else:
        t, s = unused
        nxt = []
        i = s
        while i != t:
            nxt.append(cur[i])
            i = (i + 1) % len(cur)
        nxt.append(cur[t])
        cur = nxt
print(len(cur))
print(*cur, sep='\n')

Buggy submission (verdict: Runtime Error):
from collections import defaultdict
import sys
input = sys.stdin.readline

N, M = map(int, input().split())
G = [[] for _ in range(N+1)]
for _ in range(M):
    A, B = map(int, input().split())
    G[A].append(B)

def dfs(v, o):
    if len(parents[v]) > 1:
        return []
    for c in G[v]:
        parents[c].add(v)

    ancestors.add(v)
    for c in G[v]:
        if c in ancestors - {o}:
            ancestors.remove(v)
            return []

    ret = []
    if any(c == o for c in G[v]):
        ret = [v]
    else:
        for c in G[v]:
            ret = dfs(c, o)
            if ret:
                ret.append(v)
                break

    ancestors.remove(v)
    for c in G[v]:
        parents[c].remove(v)
    return ret

for v in range(1, N+1):
    ancestors = set()
    parents = defaultdict(set)
    cand = dfs(v, v)
    if cand:
        print(len(cand))
        print(*cand, sep='\n')
        break
else:
    print(-1)

Buggy submission (verdict: Runtime Error):
from collections import defaultdict
import sys
input = sys.stdin.readline

N, M = map(int, input().split())
G = [[] for _ in range(N+1)]
for _ in range(M):
    A, B = map(int, input().split())
    G[A].append(B)

def dfs(v, o):
    if len(parents[v]) > 1:
        return []
    for c in G[v]:
        parents[c].add(v)

    ancestors.add(v)
    for c in G[v]:
        if c in ancestors - {o}:
            ancestors.remove(v)
            return []

    if any(c == o for c in G[v]):
        ret = [v]
    else:
        for c in G[v]:
            ret = dfs(c, o)
            if ret:
                ret.append(v)
                break

    ancestors.remove(v)
    for c in G[v]:
        parents[c].remove(v)
    return ret

for v in range(1, N+1):
    ancestors = set()
    parents = defaultdict(set)
    cand = dfs(v, v)
    if cand:
        print(len(cand))
        print(*cand, sep='\n')
        break
else:
    print(-1)