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


N,M = list(map(int,input().split()))
Network = UnionFind(N)

out = 0
for i in range(M):
    a, b = list(map(int,input().split()))
    Network.union(a-1, b-1)

out = 0
for L in Network.all_group_members().values():
    out = max(out,len(L))
print(out)

Fixed submission:
N,M = list(map(int,input().split()))
from collections import deque

G = [[] for i in range(N)]
for i in range(M):
    A,B=list(map(int,input().split()))
    G[A-1].append(B-1)
    G[B-1].append(A-1)

Ls = []
dist = [0]*N
for i in range(N):
    if dist[i]==1:
        continue
    s = i
    que = deque([s])
    dist[s] = 1
    L = [s]
    while que:
        v = que.popleft()
        for w in G[v]:
            if dist[w]==1:
                continue
            dist[w] = 1
            que.append(w)
            L.append(w)
    Ls.append(L)
out = 0
for L in Ls:
    out=max(len(L),out)
print(out)

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


N,M = list(map(int,input().split()))
Network = UnionFind(N)

out = 0
for i in range(M):
    a, b = list(map(int,input().split()))
    Network.union(a-1, b-1)

out = 0
for L in Network.all_group_members().values():
    out = max(out,len(L))
print(out)

Fixed submission:
N,M = list(map(int,input().split()))
from collections import deque

G = [[] for i in range(N)]
for i in range(M):
    A,B=list(map(int,input().split()))
    G[A-1].append(B-1)
    G[B-1].append(A-1)

Ls = []
dist = [0]*N
for i in range(N):
    if dist[i]==1:
        continue
    s = i
    que = deque([s])
    dist[s] = 1
    L = [s]
    while que:
        v = que.popleft()
        for w in G[v]:
            if dist[w]==1:
                continue
            dist[w] = 1
            que.append(w)
            L.append(w)
    Ls.append(L)
out = 0
for L in Ls:
    out=max(len(L),out)
print(out)

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


N,M = list(map(int,input().split()))
Network = UnionFind(N)
Friend = [0] * N
BlockInNetwork = [0] * N

for i in range(M):
    a, b = list(map(int,input().split()))
    Network.union(a-1, b-1)

out = 0
for L in Network.all_group_members().values():
    out = max(out,len(L))
print(out)

Fixed submission:
N,M = list(map(int,input().split()))
from collections import deque

G = [[] for i in range(N)]
for i in range(M):
    A,B=list(map(int,input().split()))
    G[A-1].append(B-1)
    G[B-1].append(A-1)

Ls = []
dist = [0]*N
for i in range(N):
    if dist[i]==1:
        continue
    s = i
    que = deque([s])
    dist[s] = 1
    L = [s]
    while que:
        v = que.popleft()
        for w in G[v]:
            if dist[w]==1:
                continue
            dist[w] = 1
            que.append(w)
            L.append(w)
    Ls.append(L)
out = 0
for L in Ls:
    out=max(len(L),out)
print(out)