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 _ in range(m):
    a,b=map(int, input().split())
    uf.union(a-1, b-1)
cnt=0
x=list(uf.all_group_members().values())
for l in x:
    cnt=max(cnt, len(l))
print(cnt)

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 _ in range(m):
    a,b=map(int, input().split())
    uf.union(a-1, b-1)
cnt=0
for i in range(n):
    cnt=max(cnt, uf.size(i))
print(cnt)

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 _ in range(m):
    a,b=map(int, input().split())
    uf.union(a-1, b-1)
cnt=0
for l in list(uf.all_group_members().values()):
    cnt=max(cnt, len(l))

print(cnt)

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 _ in range(m):
    a,b=map(int, input().split())
    uf.union(a-1, b-1)
cnt=0
for i in range(n):
    cnt=max(cnt, uf.size(i))
print(cnt)

Buggy python submission (verdict: Runtime Error):
from collections import Counter
n,m=map(int, input().split())
ans=1
l=[]
x=[-1]*n
for i in range(m):
    a,b=map(int, input().split())
    a-=1
    b-=1
    if x[a]+x[b]==-2:
        l.append(2)
        x[a]=len(l)-1
        x[b]=len(l)-1
    elif x[a]==x[b]:
        continue
    elif x[a]!=-1:
        l[x[b]]+=1
        x[a]=x[b]
    elif x[b]!=-1:
        l[x[a]]+=1
        x[b]=x[a]
    else:
        l[x[a]]+=l[x[b]]
        l[x[b]]=0
        for i in range(n):
            if x[i]==x[b]:
                x[i]=x[a]
print(max(l))

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 _ in range(m):
    a,b=map(int, input().split())
    uf.union(a-1, b-1)
cnt=0
for i in range(n):
    cnt=max(cnt, uf.size(i))
print(cnt)

Buggy python submission (verdict: Runtime Error):
from collections import Counter
import collections
n, m = map(int, input().split())
koseki = [-1 for _ in range(n)]  # -1
group = []
mouyatta = Counter()
for i in range(m):
    a, b = map(int, input().split())
    # if mouyatta[tuple(sorted([a,b]))]==0:
    # mouyatta[tuple(sorted([a,b]))]+=1
    if koseki[a-1] == koseki[b-1] and koseki[a-1] != -1:
        continue
    elif koseki[a-1] != -1:
        koseki[b-1] = koseki[a-1]
        group[koseki[a-1]] += 1
    elif koseki[b-1] != -1:
        koseki[a-1] = koseki[b-1]
        group[koseki[b-1]] += 1
    else:
        group.append(2)
        koseki[a-1] = len(group)-1
        koseki[b-1] = len(group)-1
# print(group)
print(max(group))


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 _ in range(m):
    a,b=map(int, input().split())
    uf.union(a-1, b-1)
cnt=0
for i in range(n):
    cnt=max(cnt, uf.size(i))
print(cnt)