Buggy python submission (verdict: Time Limit Exceeded):
n,m = map(int, input().split())
if m == 0:
  print(1)
elif m == 1:
  print(2)
else:
  ls=[]
  a,b = map(int, input().split())
  tmp = {a, b}
  ls.append(tmp)
  for i in range(m-1):
    a,b = map(int, input().split())
    for s in ls:
      if (a in s) or (b in s):
        s.add(a)
        s.add(b)
        break
      else:
        tmp = {a,b}
        ls.append(tmp)

  ans = 0
  for s in ls:
    ans = max(ans, len(s))
  print(ans)

Fixed submission:
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 = map(int, input().split())
uf = UnionFind(n)
for i in range(m):
  a,b = map(int, input().split())
  uf.union(a-1, b-1)

ans = 0
for i in uf.roots():
  ans = max(ans, uf.size(i))
print(ans)

Buggy python submission (verdict: Runtime Error):
n,m = map(int, input().split())
ls=[]
a,b = map(int, input().split())
tmp = {a, b}
ls.append(tmp)
for i in range(m-1):
  a,b = map(int, input().split())
  for s in ls:
    if (a in s) or (b in s):
      s.add(a)
      s.add(b)
      break
    else:
      tmp = {a,b}
      ls.append(tmp)

ans = 0
for s in ls:
  ans = max(ans, len(s))
print(ans)

Fixed submission:
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 = map(int, input().split())
uf = UnionFind(n)
for i in range(m):
  a,b = map(int, input().split())
  uf.union(a-1, b-1)

ans = 0
for i in uf.roots():
  ans = max(ans, uf.size(i))
print(ans)