813C - The Tag Game - CodeForces Solution

import sys
from collections import deque

toks = (tok for tok in sys.stdin.read().split())


N = int(next(toks))
X = int(next(toks))-1

adj = [[] for _ in range(N)]

for _ in range(N-1):
        v1 = int(next(toks))-1
    v2 = int(next(toks))-1
    
        adj[v1].append(v2)
    adj[v2].append(v1)


    
def find_dsts(st):
    dsts = [None for _ in range(N)]

        bfs_q = deque()
    bfs_q.append(st)
        dsts[st] = 0

    while len(bfs_q) > 0:
        cur_v = bfs_q.popleft()

                for next_v in adj[cur_v]:
                                    if dsts[next_v] == None:
                                                                dsts[next_v] = dsts[cur_v]+1
                bfs_q.append(next_v)
                
    return dsts

dsts_from_alice = find_dsts(0)
dsts_from_bob = find_dsts(X)

answer = 2*dsts_from_alice[X]
vis = [False for _ in range(N)]

bfs_q = deque()
bfs_q.append(X)
vis[X] = True

while len(bfs_q) > 0:
    cur_v = bfs_q.popleft()

    for next_v in adj[cur_v]:
        if vis[next_v]: continue

                                        if dsts_from_alice[next_v] < dsts_from_bob[next_v]:
            answer = max(answer, 2*dsts_from_bob[next_v]-1)
                                        elif dsts_from_alice[next_v] == dsts_from_bob[next_v]:
            answer = max(answer, 2*dsts_from_bob[next_v])
                                                else:
            answer = max(answer, 2*dsts_from_alice[next_v])
                                    vis[next_v] = True
            bfs_q.append(next_v)

print(answer)