Buggy python submission (verdict: Time Limit Exceeded):
N,M = map(int,input().split())
ls = [0 for _ in range(N)]
flg = 1
for i in range(M):
    a,b = map(int,input().split())
    if ls[a-1] == 0 and ls[b-1] == 0:
        ls[a-1]=flg
        ls[b-1]=flg
        flg+=1
    elif ls[a-1] == 0:
        ls[a-1] = ls[b-1]
    elif ls[b-1] == 0:
        ls[b-1] = ls[a-1]
    else:
        check = ls[b-1]
        ls[b-1] = ls[a-1]
        for l in range(len(ls)):
            if ls[l] == check:
                ls[l]=ls[a-1]
import collections
c = collections.Counter(ls)
plan = c.most_common()
ans = 0
if plan[0][0] == 0 and M != 0:
    ans = plan[1][1]
elif M ==0:
    ans =1
else:
    ans = plan[0][1]
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 j in range(n):
    check = uf.size(j)
    if ans < check:
        ans = check
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
N,M = map(int,input().split())
ls = [0 for _ in range(N)]
flg = 1
for i in range(M):
    a,b = map(int,input().split())
    if ls[a-1] == 0 and ls[b-1] == 0:
        ls[a-1]=flg
        ls[b-1]=flg
        flg+=1
    elif ls[a-1] == 0:
        ls[a-1] = ls[b-1]
    elif ls[b-1] == 0:
        ls[b-1] = ls[a-1]
    else:
        x = ls.count(ls[a-1])
        y = ls.count(ls[b-1])
        if x >= y:
            check = ls[a-1]
        else:
            check = ls[b-1]
        ls[a-1] = check
        ls[b-1] = check
import collections
c = collections.Counter(ls)
plan = c.most_common()
ans = 0
if plan[0][0] == 0 and M != 0:
    ans = plan[1][1]
elif M ==0:
    ans =1    
else:
    ans = plan[0][1]
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 j in range(n):
    check = uf.size(j)
    if ans < check:
        ans = check
print(ans)

Buggy python submission (verdict: Runtime Error):
N,M = map(int,input().split())
ls = [0 for _ in range(N)]
flg = 1
for i in range(M):
    a,b = map(int,input().split())
    if ls[a-1] == 0 and ls[b-1]==0:
        ls[a-1]=flg
        ls[b-1]=flg
        flg+=1
    elif ls[a-1] == 0:
        ls[a-1] = ls[b-1]
    elif ls[b-1] == 0:
        ls[b-1] = ls[a-1]
    else:
        check = min(ls[a-1],ls[b-1])
        ls[a-1] = check
        ls[b-1] = check
import collections
c = collections.Counter(ls)
plan = c.most_common()
ans = 0
if plan[0][0] == 0:
    ans = plan[1][1]
else:
    ans = plan[0][1]
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 j in range(n):
    check = uf.size(j)
    if ans < check:
        ans = check
print(ans)

Buggy python submission (verdict: Runtime Error):
N,M = map(int,input().split())
ls = [0 for _ in range(N)]
flg = 1
for i in range(M):
    a,b = map(int,input().split())
    if ls[a-1] == 0 and ls[b-1]==0:
        ls[a-1]=flg
        ls[b-1]=flg
        flg+=1
    elif ls[a-1] == 0:
        ls[a-1] = ls[b-1]
    elif ls[b-1] == 0:
        ls[b-1] = ls[a-1]
    else:
        check = min(ls[a-1],ls[b-1])
        ls[a-1] = check
        ls[b-1] = check
from statistics import mode
com = mode(ls)
print(ls.count(com))       

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 j in range(n):
    check = uf.size(j)
    if ans < check:
        ans = check
print(ans)