Buggy python submission (verdict: Time Limit Exceeded):
import sys

class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n

    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]

    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)

        if x == y:
            return

        if self.parents[x] > self.parents[y]:
            x, y = y, x

        self.parents[x] += self.parents[y]
        self.parents[y] = x

    def size(self, x):
        return -self.parents[self.find(x)]

    def same(self, x, y):
        return self.find(x) == self.find(y)

    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]

    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]

    def group_count(self):
        return len(self.roots())

    def all_group_members(self):
        return {r: self.members(r) for r in self.roots()}

    def __str__(self):
        return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())

input = sys.stdin.readline

N, M = map(int, input().split())
relations = []
for _ in range(M):
    a, b = map(int, input().split())
    relations.append((a-1, b-1))

uf = UnionFind(N)
for a, b in relations:
    uf.union(a, b)

max_n = 0
# print(uf.all_group_members())
for i, members in uf.all_group_members().items():
    # print(members)
    max_n = max(max_n, len(members))

print(max_n)

Fixed submission:
import sys
from collections import deque

input = sys.stdin.readline

N, M = map(int, input().split())
relations = [[] for _ in range(N)]
for _ in range(M):
    a, b = map(int, input().split())
    a -= 1
    b -= 1
    relations[a].append(b)
    relations[b].append(a)

ans = 0
visited = set()
for i in range(N):
    q = deque()
    q.append(i)
    start = len(visited)
    while q:
        pos = q.popleft()
        if pos in visited:
            continue
        visited.add(pos)
        for n_pos in relations[pos]:
            if n_pos in visited:
                continue
            q.appendleft(n_pos)
    end = len(visited)
    ans = max(ans, end - start)

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import sys

class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n

    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]

    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)

        if x == y:
            return

        if self.parents[x] > self.parents[y]:
            x, y = y, x

        self.parents[x] += self.parents[y]
        self.parents[y] = x

    def size(self, x):
        return -self.parents[self.find(x)]

    def same(self, x, y):
        return self.find(x) == self.find(y)

    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]

    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]

    def group_count(self):
        return len(self.roots())

    def all_group_members(self):
        return {r: self.members(r) for r in self.roots()}

    def __str__(self):
        return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())

input = sys.stdin.readline

N, M = map(int, input().split())
relations = []
for _ in range(M):
    a, b = map(int, input().split())
    relations.append((a-1, b-1))

uf = UnionFind(N)
for a, b in relations:
    uf.union(a, b)

max_n = 0
for i in range(uf.group_count()):
    max_n = max(max_n, len(uf.members(i)))

print(max_n)

Fixed submission:
import sys
from collections import deque

input = sys.stdin.readline

N, M = map(int, input().split())
relations = [[] for _ in range(N)]
for _ in range(M):
    a, b = map(int, input().split())
    a -= 1
    b -= 1
    relations[a].append(b)
    relations[b].append(a)

ans = 0
visited = set()
for i in range(N):
    q = deque()
    q.append(i)
    start = len(visited)
    while q:
        pos = q.popleft()
        if pos in visited:
            continue
        visited.add(pos)
        for n_pos in relations[pos]:
            if n_pos in visited:
                continue
            q.appendleft(n_pos)
    end = len(visited)
    ans = max(ans, end - start)

print(ans)