트리 트리오 중간값

https://school.programmers.co.kr/learn/courses/30/lessons/68937

def solution(n, edges):
    def bfs(first_node):
        queue = deque([(first_node, 0)])
        visited = {first_node: 0}
        max_dist = 0
        while queue:
            node, distance = queue.popleft()
            for next_node in tree[node]:
                if next_node not in visited:
                    visited[next_node] = distance + 1
                    queue.append((next_node, distance + 1))
                    max_dist = max(max_dist, distance + 1)
        return max_dist, [k for k, v in visited.items() if v == max_dist]

    def solve():
        _, some = bfs(1)
        max_dist, others = bfs(some[0])
        if len(others) >= 2:
            return max_dist
        else:
            max_dist, others = bfs(others[0])
            if len(others) >= 2:
                return max_dist
            else:
                return max_dist - 1

    from collections import defaultdict, deque
    tree = defaultdict(list)
    for a, b in edges:
        tree[a].append(b)
        tree[b].append(a)
    return solve()


print(solution(4, [[1, 2], [2, 3], [3, 4]]))
print(solution(5, [[1, 5], [2, 5], [3, 5], [4, 5]]))