Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
# -*- coding:utf-8 -*-

from collections import defaultdict
import json
from copy import deepcopy

def dfs(graph, s, d, count=0):
  if len(graph[s]) == 0:
    d[s] = count
  for t in graph[s]:
    if s in graph[t]:
      graph[t].remove(s)
    dfs(graph, t, d, count+1)

if __name__=='__main__':
  graph = defaultdict(list)
  n, u, v = map(int, input().strip().split())
  for _ in range(n-1):
    ai, bi = map(int, input().strip().split())
    graph[ai].append(bi)
    graph[bi].append(ai)
  du, dv = dict(), dict()
  du[u], dv[v] = 0, 0
  dfs(deepcopy(graph), u, du)
  dfs(deepcopy(graph), v, dv)

  costv = [0]
  for k in dv.keys():
    if k in du and du[k] < dv[k]:
      costv.append(dv[k])
  print(max(costv) - 1)

Fixed submission:
#!/usr/bin/env python3
# -*- coding:utf-8 -*-

from collections import defaultdict
import json
from copy import deepcopy
import sys
sys.setrecursionlimit(10**4)

def dfs(graph, s):
  dist = [-1 for _ in range(len(graph))]
  dist[s] = 0
  stack = [s]
  while len(stack) != 0:
    s = stack.pop()
    for t in graph[s]:
      dist[t] = dist[s]+1
      if s in graph[t]:
        graph[t].remove(s)
      stack.append(t)
  return dist

if __name__=='__main__':
  graph = defaultdict(list)
  n, u, v = map(int, input().strip().split())
  for _ in range(n-1):
    ai, bi = map(int, input().strip().split())
    graph[ai-1].append(bi-1)
    graph[bi-1].append(ai-1)
  du = dfs(deepcopy(graph), u-1)
  dv = dfs(deepcopy(graph), v-1)
  costv = [0]
  for duv, dvv in zip(du, dv):
    if duv < dvv:
      costv.append(dvv)
  print(max(costv) - 1)

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
# -*- coding:utf-8 -*-

from collections import defaultdict
import json
from copy import deepcopy

def dfs(graph, s, d, count=0, visited_nodes=None):
  if visited_nodes is None:
    visited_nodes = []
  visited_nodes.append(s)
  if len(graph[s]) == 1:
    d[s] = count
  for t in graph[s]:
    if t in visited_nodes:
      continue
    dfs(graph, t, d, count+1, visited_nodes)

if __name__=='__main__':
  graph = defaultdict(list)
  n, u, v = map(int, input().strip().split())
  for _ in range(n-1):
    ai, bi = map(int, input().strip().split())
    graph[ai].append(bi)
    graph[bi].append(ai)
  du, dv = dict(), dict()
  du[u], dv[v] = 0, 0
  dfs(graph, u, du)
  dfs(graph, v, dv)

  costv = []
  for k in dv.keys():
    if k in du and du[k] < dv[k]:
      costv.append(dv[k])
  print(max(costv) - 1)

Fixed submission:
#!/usr/bin/env python3
# -*- coding:utf-8 -*-

from collections import defaultdict
import json
from copy import deepcopy
import sys
sys.setrecursionlimit(10**4)

def dfs(graph, s):
  dist = [-1 for _ in range(len(graph))]
  dist[s] = 0
  stack = [s]
  while len(stack) != 0:
    s = stack.pop()
    for t in graph[s]:
      dist[t] = dist[s]+1
      if s in graph[t]:
        graph[t].remove(s)
      stack.append(t)
  return dist

if __name__=='__main__':
  graph = defaultdict(list)
  n, u, v = map(int, input().strip().split())
  for _ in range(n-1):
    ai, bi = map(int, input().strip().split())
    graph[ai-1].append(bi-1)
    graph[bi-1].append(ai-1)
  du = dfs(deepcopy(graph), u-1)
  dv = dfs(deepcopy(graph), v-1)
  costv = [0]
  for duv, dvv in zip(du, dv):
    if duv < dvv:
      costv.append(dvv)
  print(max(costv) - 1)

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
# -*- coding:utf-8 -*-

from collections import defaultdict
import json
from copy import deepcopy

def dfs(graph, s, d, count=0):
  if len(graph[s]) == 0:
    # print(count)
    d[s] = count
  for t in graph[s]:
    if s in graph[t]:
      graph[t].remove(s)
    # print(s, t)
    dfs(graph, t, d, count+1)

if __name__=='__main__':
  graph = defaultdict(list)
  n, u, v = map(int, input().strip().split())
  for _ in range(n-1):
    ai, bi = map(int, input().strip().split())
    graph[ai].append(bi)
    graph[bi].append(ai)
  du, dv = dict(), dict()
  du[u], dv[v] = 0, 0
  dfs(deepcopy(graph), u, du)
  dfs(deepcopy(graph), v, dv)
  # print(json.dumps(du, indent=2))
  # print(json.dumps(dv, indent=2))

  costv = []
  for k in dv.keys():
    if k in du and du[k] < dv[k]:
      costv.append(dv[k])
  print(max(costv) - 1)

Fixed submission:
#!/usr/bin/env python3
# -*- coding:utf-8 -*-

from collections import defaultdict
import json
from copy import deepcopy
import sys
sys.setrecursionlimit(10**4)

def dfs(graph, s):
  dist = [-1 for _ in range(len(graph))]
  dist[s] = 0
  stack = [s]
  while len(stack) != 0:
    s = stack.pop()
    for t in graph[s]:
      dist[t] = dist[s]+1
      if s in graph[t]:
        graph[t].remove(s)
      stack.append(t)
  return dist

if __name__=='__main__':
  graph = defaultdict(list)
  n, u, v = map(int, input().strip().split())
  for _ in range(n-1):
    ai, bi = map(int, input().strip().split())
    graph[ai-1].append(bi-1)
    graph[bi-1].append(ai-1)
  du = dfs(deepcopy(graph), u-1)
  dv = dfs(deepcopy(graph), v-1)
  costv = [0]
  for duv, dvv in zip(du, dv):
    if duv < dvv:
      costv.append(dvv)
  print(max(costv) - 1)

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
# -*- coding:utf-8 -*-

from collections import defaultdict
import json
from copy import deepcopy

def dfs(graph, s, d, count=0):
  if len(graph[s]) == 0:
    d[s] = count
  for t in graph[s]:
    if s in graph[t]:
      graph[t].remove(s)
    dfs(graph, t, d, count+1)

if __name__=='__main__':
  graph = defaultdict(list)
  n, u, v = map(int, input().strip().split())
  for _ in range(n-1):
    ai, bi = map(int, input().strip().split())
    graph[ai].append(bi)
    graph[bi].append(ai)
  du, dv = dict(), dict()
  du[u], dv[v] = 0, 0
  dfs(deepcopy(graph), u, du)
  dfs(deepcopy(graph), v, dv)

  costv = []
  for k in dv.keys():
    if du[k] < dv[k]:
      costv.append(dv[k])
  print(max(costv) - 1)

Fixed submission:
#!/usr/bin/env python3
# -*- coding:utf-8 -*-

from collections import defaultdict
import json
from copy import deepcopy
import sys
sys.setrecursionlimit(10**4)

def dfs(graph, s):
  dist = [-1 for _ in range(len(graph))]
  dist[s] = 0
  stack = [s]
  while len(stack) != 0:
    s = stack.pop()
    for t in graph[s]:
      dist[t] = dist[s]+1
      if s in graph[t]:
        graph[t].remove(s)
      stack.append(t)
  return dist

if __name__=='__main__':
  graph = defaultdict(list)
  n, u, v = map(int, input().strip().split())
  for _ in range(n-1):
    ai, bi = map(int, input().strip().split())
    graph[ai-1].append(bi-1)
    graph[bi-1].append(ai-1)
  du = dfs(deepcopy(graph), u-1)
  dv = dfs(deepcopy(graph), v-1)
  costv = [0]
  for duv, dvv in zip(du, dv):
    if duv < dvv:
      costv.append(dvv)
  print(max(costv) - 1)