Snakeless path

import sys
from collections import deque


def main():
    input = list(map(int, sys.stdin.buffer.read().split()))
    n, m = input[0], input[1]

    s = [set() for _ in range(n + 1)]
    j = 2
    for _ in range(m):
        a, b = input[j], input[j + 1]
        s[a].add(b)
        s[b].add(a)
        j+=2

    parent = [0] * (n + 1)
    parent[1] = -1
    queue = deque([1])

    next_vertex = list(range(1, n + 2))
    previous_vertex = list(range(-1, n))
    head = 2
    next_vertex[0] = 0
    previous_vertex[head] = 0

    while queue and parent[n] == 0:
        current = queue.popleft()
        vertex = head

        while vertex <= n:
            next_unvisited = next_vertex[vertex]
            if vertex not in s[current]:
                parent[vertex] = current
                queue.append(vertex)

                if previous_vertex[vertex] == 0:
                    head = next_unvisited
                else:
                    next_vertex[previous_vertex[vertex]] = next_unvisited
                if next_unvisited <= n:
                    previous_vertex[next_unvisited] = previous_vertex[vertex]

            vertex = next_unvisited

    if parent[n] == 0:
        print(0)
        return

    path = []
    vertex = n
    while vertex != -1:
        path.append(vertex)
        vertex = parent[vertex]
    path.reverse()

    print(len(path))
    print(*path)


if __name__ == "__main__":
    main()

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