Buggy python submission (verdict: Time Limit Exceeded):
# -*- 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(10000)


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


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

    path = deque([1])
    path.append(g[1][0])
    while not (set(g[path[0]]) | set(g[path[-1]])).issubset(set(path)):
        l = set(g[path[0]]) - set(path)
        if l:
            path.appendleft(list(l)[0])
            continue
        r = set(g[path[-1]]) - set(path)
        if r:
            path.append(list(r)[0])
            continue

    print(len(path))
    print(' '.join(map(lambda x: str(x), path)))


def main():
    # N = read_int()
    N, M = read_int_n()
    AB = [read_int_n() for _ in range(M)]
    (slv(N, M, AB))


if __name__ == '__main__':
    main()


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(10000)


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


@mt
def slv(N, M, AB):
    g = defaultdict(set)
    for a, b in AB:
        g[a].add(b)
        g[b].add(a)

    path = deque([1])
    path.append(list(g[1])[0])
    path_s = set(path)
    while not (g[path[0]] | g[path[-1]]).issubset(path_s):
        l = g[path[0]] - path_s
        if l:
            for v in l:
                path.appendleft(v)
                path_s.add(v)
                break
            continue
        r = g[path[-1]] - path_s
        if r:
            for v in r:
                path.append(v)
                path_s.add(v)
                break
            continue

    print(len(path))
    print(' '.join(map(lambda x: str(x), path)))


def main():
    # N = read_int()
    N, M = read_int_n()
    AB = [read_int_n() for _ in range(M)]
    (slv(N, M, AB))


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
# -*- 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(10000)


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


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

    path = deque([1])
    path.append(g[1][0])
    while not (set(g[path[0]]) | set(g[path[-1]])).issubset(set(path)):
        l = set(g[path[0]]) - set(path)
        if l:
            path.appendleft(list(l)[0])
            continue
        r = set(g[path[-1]]) - set(path)
        if r:
            path.append(list(r)[0])
            continue

    print(len(path))
    print(' '.join(map(lambda x: str(x), path)))


def main():
    # N = read_int()
    N, M = read_int_n()
    AB = [read_int_n() for _ in range(M)]
    (slv(N, M, AB))


if __name__ == '__main__':
    main()


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(10000)


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


@mt
def slv(N, M, AB):
    g = defaultdict(set)
    for a, b in AB:
        g[a].add(b)
        g[b].add(a)

    path = deque([1])
    path.append(list(g[1])[0])
    path_s = set(path)
    while not (g[path[0]] | g[path[-1]]).issubset(path_s):
        l = g[path[0]] - path_s
        if l:
            for v in l:
                path.appendleft(v)
                path_s.add(v)
                break
            continue
        r = g[path[-1]] - path_s
        if r:
            for v in r:
                path.append(v)
                path_s.add(v)
                break
            continue

    print(len(path))
    print(' '.join(map(lambda x: str(x), path)))


def main():
    # N = read_int()
    N, M = read_int_n()
    AB = [read_int_n() for _ in range(M)]
    (slv(N, M, AB))


if __name__ == '__main__':
    main()