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 = 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 range(n):
  ans = max(ans, len(uf.members(i)))

print(ans)

Fixed submission:
def find(x):
    if par[x] < 0:
        return x
    else:
        par[x] = find(par[x])
        return par[x]
#xとyの属する集合の併合
def unite(x,y):
    x = find(x)
    y = find(y)
    
    if x == y:
        return False
    else:
        #sizeの大きいほうがx
        if par[x] > par[y]:
            x,y = y,x
        par[x] += par[y]
        par[y] = x
        return True

#xとyが同じ集合に属するかの判定
def same(x,y):
    return find(x) == find(y)

#xが属する集合の個数
def size(x):
    return -par[find(x)]

#初期化
#根なら-size,子なら親の頂点
n, m = map(int, input().split())
par = [-1]*n

for i in range(m):
  a, b = map(int, input().split())
  unite(a - 1, b - 1)

ans = 0
for i in range(n):
  ans = max(ans, size(i))

print(ans)

Buggy python submission (verdict: Runtime Error):
n, m = map(int, input().split())
group_idx = [i for i in range(n)]

for i in range(m):
  a, b = map(int, input().split())
  if a > b:
    a, b = b, a
  group_idx[b - 1] = group_idx[a - 1]
  print(group_idx)

groups = [0 for i in range(n)]

for i in range(n):
  groups[group_idx[i]] += 1

print(group_idx)
print(max(groups))

Fixed submission:
def find(x):
    if par[x] < 0:
        return x
    else:
        par[x] = find(par[x])
        return par[x]
#xとyの属する集合の併合
def unite(x,y):
    x = find(x)
    y = find(y)
    
    if x == y:
        return False
    else:
        #sizeの大きいほうがx
        if par[x] > par[y]:
            x,y = y,x
        par[x] += par[y]
        par[y] = x
        return True

#xとyが同じ集合に属するかの判定
def same(x,y):
    return find(x) == find(y)

#xが属する集合の個数
def size(x):
    return -par[find(x)]

#初期化
#根なら-size,子なら親の頂点
n, m = map(int, input().split())
par = [-1]*n

for i in range(m):
  a, b = map(int, input().split())
  unite(a - 1, b - 1)

ans = 0
for i in range(n):
  ans = max(ans, size(i))

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
n, m = map(int, input().split())
friends = [0 for i in range(n)]
relation = [[False for i in range(n)] for j in range(n)]

for i in range(m):
  a, b = map(int, input().split())
  if not relation[a - 1][b - 1]:
    relation[a - 1][b - 1] = True
    relation[b - 1][a - 1] = True
    friends[a - 1] += 1
    friends[b - 1] += 1

print(max(friends) + 1)

Fixed submission:
def find(x):
    if par[x] < 0:
        return x
    else:
        par[x] = find(par[x])
        return par[x]
#xとyの属する集合の併合
def unite(x,y):
    x = find(x)
    y = find(y)
    
    if x == y:
        return False
    else:
        #sizeの大きいほうがx
        if par[x] > par[y]:
            x,y = y,x
        par[x] += par[y]
        par[y] = x
        return True

#xとyが同じ集合に属するかの判定
def same(x,y):
    return find(x) == find(y)

#xが属する集合の個数
def size(x):
    return -par[find(x)]

#初期化
#根なら-size,子なら親の頂点
n, m = map(int, input().split())
par = [-1]*n

for i in range(m):
  a, b = map(int, input().split())
  unite(a - 1, b - 1)

ans = 0
for i in range(n):
  ans = max(ans, size(i))

print(ans)