AtCoder Beginner Contest 177 [ D - Friends ] https://atcoder.jp/contests/abc177/tasks/abc177_d

abc177_d.py

'''
[問題]
https://atcoder.jp/contests/abc177/tasks/abc177_d

[解説]
https://atcoder.jp/contests/abc177/editorial/90

[参考]
AtCoder Beginner Contest 206（Sponsored by Panasonic） [ D - KAIBUNsyo ]をPythonでUnionFindを使って解く
https://uchidama.hatenablog.com/entry/2021/06/22/170804

PythonでのUnion-Find（素集合データ構造）の実装と使い方
https://note.nkmk.me/python-union-find/

[結果]
PyPy3(7.3.0) AC 304ms
Python(3.8.2) AC 510ms
'''

import sys
from collections import defaultdict

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):
        group_members = defaultdict(list)
        for member in range(self.n):
            group_members[self.find(member)].append(member)
        return group_members

    def __str__(self):
        return '\n'.join(f'{r}: {m}' for r, m in self.all_group_members().items())



sys.setrecursionlimit(10 ** 6)  # 再帰上限の引き上げ
input = sys.stdin.readline

INF = 2 ** 63 - 1

N, M = map(int, input().split())

# UnionFindで木構造を作成
uf = UnionFind(N + 1)

for i in range(M):
    A, B = map(int, input().split())
    # UnionFindに友達関係を入力
    uf.union(A, B)

ans = 0
for r, m in uf.all_group_members().items():
    if m[0] == 0:
        continue

    # 友達集合の最大を求める。最大の友達集合を一人一人分割してしまえば、全てバラける
    ans = max(ans, len(m))
    #print(r, m) # 木構造の確認、デバッグ用

print(ans)