图的广度优先搜索BFS

def solve():
    from collections import deque
    graph = {____: [____, ____], ____: [____, ____], ____: [____], ____: [____]}
    visited = []
    q = deque(____)
    seen = {____}
    while q:
        node = q.popleft()
        visited.append(____)
        for neighbor in graph[node]:
            if neighbor not in seen:
                seen.add(____)
                q.append(____)
    print('____'.join(map(____)))