Buggy python submission (verdict: Runtime Error):
import sys


def dfs(v, parent, depth):
    parents[v] = parent
    self_depths[v] = depth
    sd = 0
    for u in links[v]:
        if u == parent:
            continue
        res = dfs(u, v, depth + 1)
        sd = max(sd, res + 1)

    subtree_depths[v] = sd
    return sd


def solve(u, v):
    ans = 0
    catch = self_depths[u] // 2
    while self_depths[u] > catch:
        ans = max(ans, self_depths[u] + subtree_depths[u] - 1)
        u = parents[u]
    return ans


n, u, v = map(int, input().split())
u -= 1
v -= 1
links = [set() for _ in range(n)]
for line in sys.stdin:
    a, b = map(int, line.split())
    a -= 1
    b -= 1
    links[a].add(b)
    links[b].add(a)

parents = [-1] * n
self_depths = [0] * n
subtree_depths = [0] * n
dfs(v, -1, 0)
# print(parents)
# print(self_depths)
# print(subtree_depths)
print(solve(u, v))


Fixed submission:
import sys

sys.setrecursionlimit(10 ** 5)


def dfs(v, parent, depth):
    parents[v] = parent
    self_depths[v] = depth
    sd = 0
    for u in links[v]:
        if u == parent:
            continue
        res = dfs(u, v, depth + 1)
        sd = max(sd, res + 1)

    subtree_depths[v] = sd
    return sd


def solve(u):
    ans = 0
    catch = self_depths[u] // 2
    while self_depths[u] > catch:
        ans = max(ans, self_depths[u] + subtree_depths[u] - 1)
        u = parents[u]
    return ans


n, u, v = map(int, input().split())
u -= 1
v -= 1
links = [set() for _ in range(n)]
for line in sys.stdin:
    a, b = map(int, line.split())
    a -= 1
    b -= 1
    links[a].add(b)
    links[b].add(a)

parents = [-1] * n
self_depths = [0] * n
subtree_depths = [0] * n
dfs(v, -1, 0)
# print(parents)
# print(self_depths)
# print(subtree_depths)
print(solve(u))

Buggy python submission (verdict: Runtime Error):
import sys


def dfs(v, parent, depth):
    parents[v] = parent
    self_depths[v] = depth
    sd = 0
    for u in links[v]:
        if u == parent:
            continue
        res = dfs(u, v, depth + 1)
        sd = max(sd, res + 1)

    subtree_depths[v] = sd
    return sd


def solve(u, v):
    ans = 0
    while u != v:
        ans = max(ans, self_depths[u] + subtree_depths[u] - 1)
        u = parents[u]
    return ans


n, u, v = map(int, input().split())
u -= 1
v -= 1
links = [set() for _ in range(n)]
for line in sys.stdin:
    a, b = map(int, line.split())
    a -= 1
    b -= 1
    links[a].add(b)
    links[b].add(a)

parents = [-1] * n
self_depths = [0] * n
subtree_depths = [0] * n
dfs(v, -1, 0)
# print(parents)
# print(self_depths)
# print(subtree_depths)
print(solve(u, v))


Fixed submission:
import sys

sys.setrecursionlimit(10 ** 5)


def dfs(v, parent, depth):
    parents[v] = parent
    self_depths[v] = depth
    sd = 0
    for u in links[v]:
        if u == parent:
            continue
        res = dfs(u, v, depth + 1)
        sd = max(sd, res + 1)

    subtree_depths[v] = sd
    return sd


def solve(u):
    ans = 0
    catch = self_depths[u] // 2
    while self_depths[u] > catch:
        ans = max(ans, self_depths[u] + subtree_depths[u] - 1)
        u = parents[u]
    return ans


n, u, v = map(int, input().split())
u -= 1
v -= 1
links = [set() for _ in range(n)]
for line in sys.stdin:
    a, b = map(int, line.split())
    a -= 1
    b -= 1
    links[a].add(b)
    links[b].add(a)

parents = [-1] * n
self_depths = [0] * n
subtree_depths = [0] * n
dfs(v, -1, 0)
# print(parents)
# print(self_depths)
# print(subtree_depths)
print(solve(u))