Buggy python submission (verdict: Time Limit Exceeded):
import sys


def input(): return sys.stdin.readline().strip()
def I(): return int(input())
def LI(): return list(map(int, input().split()))
def IR(n): return [I() for i in range(n)]
def LIR(n): return [LI() for i in range(n)]
def SR(n): return [S() for i in range(n)]
def S(): return input()
def LS(): return input().split()


class UnionFind():
    def __init__(self, n):
        self.n = n
        # 各要素の親要素 親は -(その集合の要素数)
        self.parents = [-1] * n

    def find(self, x):
        '''要素 x の親要素の番号を返す'''
        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):
        '''要素 x が属する集合と要素 y が属する集合を併合する'''
        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):
        '''要素 x が属する集合の要素数を返す'''
        return -self.parents[self.find(x)]

    def same(self, x, y):
        '''要素 x と要素 y が同じ集合に属するかを返す'''
        return self.find(x) == self.find(y)

    def members(self, x):
        '''要素 x が属する集合の要素をリストを返す'''
        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())


def main():
    n, m = LI()
    uf = UnionFind(n)
    for _ in range(m):
        a, b, = LI()
        a, b = a-1, b-1
        uf.union(a, b)
    ans = 0
    for _, v in uf.all_group_members().items():
        ans = max(ans, len(v))
    print(ans)


main()


Fixed submission:
import sys

sys.setrecursionlimit(10**7)


def input(): return sys.stdin.readline().strip()
def I(): return int(input())
def LI(): return list(map(int, input().split()))
def IR(n): return [I() for i in range(n)]
def LIR(n): return [LI() for i in range(n)]
def SR(n): return [S() for i in range(n)]
def S(): return input()
def LS(): return input().split()


n, m = LI()

root = [-1] * n


def r(x):
    if root[x] < 0:
        return x
    else:
        root[x] = r(root[x])
        return root[x]


def unite(x, y):
    x = r(x)
    y = r(y)
    if x == y:
        return
    root[x] += root[y]
    root[y] = x


def size(x):
    x = r(x)
    return -root[x]


for _ in range(m):
    a, b = LI()
    a -= 1
    b -= 1
    unite(a, b)

ans = max(map(size, range(n)))
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import sys


def input(): return sys.stdin.readline().strip()
def I(): return int(input())
def LI(): return list(map(int, input().split()))
def IR(n): return [I() for i in range(n)]
def LIR(n): return [LI() for i in range(n)]
def SR(n): return [S() for i in range(n)]
def S(): return input()
def LS(): return input().split()


class UnionFind():
    def __init__(self, n):
        self.n = n
        # 各要素の親要素 親は -(その集合の要素数)
        self.parents = [-1] * n

    def find(self, x):
        '''要素 x の親要素の番号を返す'''
        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):
        '''要素 x が属する集合と要素 y が属する集合を併合する'''
        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):
        '''要素 x が属する集合の要素数を返す'''
        return -self.parents[self.find(x)]

    def same(self, x, y):
        '''要素 x と要素 y が同じ集合に属するかを返す'''
        return self.find(x) == self.find(y)

    def members(self, x):
        '''要素 x が属する集合の要素をリストを返す'''
        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())


def main():
    n, m = LI()
    uf = UnionFind(n)
    for _ in range(m):
        a, b, = LI()
        a, b = a-1, b-1
        uf.union(a, b)
    ans = 0
    for _, v in uf.all_group_members().items():
        ans = max(ans, len(v))
    print(ans)


main()


Fixed submission:
import sys

sys.setrecursionlimit(10**7)


def input(): return sys.stdin.readline().strip()
def I(): return int(input())
def LI(): return list(map(int, input().split()))
def IR(n): return [I() for i in range(n)]
def LIR(n): return [LI() for i in range(n)]
def SR(n): return [S() for i in range(n)]
def S(): return input()
def LS(): return input().split()


n, m = LI()

root = [-1] * n


def r(x):
    if root[x] < 0:
        return x
    else:
        root[x] = r(root[x])
        return root[x]


def unite(x, y):
    x = r(x)
    y = r(y)
    if x == y:
        return
    root[x] += root[y]
    root[y] = x


def size(x):
    x = r(x)
    return -root[x]


for _ in range(m):
    a, b = LI()
    a -= 1
    b -= 1
    unite(a, b)

ans = max(map(size, range(n)))
print(ans)