Buggy python submission (verdict: Time Limit Exceeded):
n,m=map(int,input().split())
import sys
sys.setrecursionlimit(10**7)

# Union FInd
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())
uf=UnionFind(n)
for _ in range(m):
  a,b=map(int,input().split())
  a,b=a-1,b-1
  uf.union(a,b)
ans=0
d=uf.all_group_members()
for c in d.values():
  ans=max(ans,len(c))
print(ans)


Fixed submission:
import sys
sys.setrecursionlimit(10**7)

N,Q = map(int,input().split())
par = [i for i in range(N)]
rank = [0]*(N+1)
def find(x):
    if par[x] == x:
        return x
    else:
        par[x] = find(par[x]) #経路圧縮
        return par[x]
def same(x,y):
    return find(x) == find(y)
def unite(x,y):
    x = find(x)
    y = find(y)
    if x == y:
        return 0
    if rank[x] < rank[y]:
        par[x] = y
    else:
        par[y] = x
        if rank[x]==rank[y]:rank[x]+=1

for _ in range(Q):
  a,b=map(int,input().split())
  a,b=a-1,b-1
  unite(a,b)
from collections import Counter
a=[-1]*N
for i in range(N):
  a[i]=find(i)
ca=Counter(a)
ans=0
for c in ca.values():
  ans=max(ans,c)
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
n,m=map(int,input().split())

# Union FInd
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())
uf=UnionFind(n)
for _ in range(m):
  a,b=map(int,input().split())
  a,b=a-1,b-1
  uf.union(a,b)
ans=0
d=uf.all_group_members()
for c in d.values():
  ans=max(ans,len(c))
print(ans)


Fixed submission:
import sys
sys.setrecursionlimit(10**7)

N,Q = map(int,input().split())
par = [i for i in range(N)]
rank = [0]*(N+1)
def find(x):
    if par[x] == x:
        return x
    else:
        par[x] = find(par[x]) #経路圧縮
        return par[x]
def same(x,y):
    return find(x) == find(y)
def unite(x,y):
    x = find(x)
    y = find(y)
    if x == y:
        return 0
    if rank[x] < rank[y]:
        par[x] = y
    else:
        par[y] = x
        if rank[x]==rank[y]:rank[x]+=1

for _ in range(Q):
  a,b=map(int,input().split())
  a,b=a-1,b-1
  unite(a,b)
from collections import Counter
a=[-1]*N
for i in range(N):
  a[i]=find(i)
ca=Counter(a)
ans=0
for c in ca.values():
  ans=max(ans,c)
print(ans)