Problem p02573 (python)

Fixed submission:
# Union Find
#0オリジン以下注意

#xの根を求める
def find(x):
    if par[x] < 0:
        return x
    else:
        par[x] = find(par[x])
        return par[x]
    
def unite(x, y):
    x = find(x)
    y = find(y)
    
    if x == y:
        return False
    else:
        #sizeの大きい方がx
        if par[x] > par[y]:
            x, y = y, x
        par[x] += par[y] #ここで集合の個数を更新している
        par[y] = x
        return True
    
#xとyが同じ集合に含まれているかどうかを判定(同じ集合にあればTrue)
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)
M = 0
for i in range(n):
    M = max(M, size(i))
print(M)

################以下にコードを書く
######2次元配列が与えられたら１次元に直す必要あり

Buggy submission (verdict: Runtime Error):
import sys
input=sys.stdin.readline
sys.setrecursionlimit(10 ** 8)
from itertools import accumulate
from itertools import permutations
from itertools import combinations
from collections import defaultdict
from collections import Counter
import fractions
import math
from collections import deque
from bisect import bisect_left
from bisect import bisect_right
from bisect import insort_left
import itertools
from heapq import heapify
from heapq import heappop
from heapq import heappush
import heapq
from copy import deepcopy
from decimal import Decimal
alf = list("abcdefghijklmnopqrstuvwxyz")
ALF = list("ABCDEFGHIJKLMNOPQRSTUVWXYZ")
#import numpy as np
INF = float("inf")
#d = defaultdict(int)
#d = defaultdict(list)
N,M = map(int,input().split())
count = [[0]*N for _ in range(N)]
fr = [[] for _ in range(N)]
for _ in range(M):
    a,b = map(int,input().split())
    a,b = a-1,b-1
    if count[a][b] == 1:
        continue
    count[a][b] = 1
    count[b][a] = 1
    fr[a].append(b)
    fr[b].append(a)
used = [-1]*N 
MAX = 0
def dfs(fr,cur,parent):
    global cnt
    children = fr[cur]
    for chi in children:
        if used[chi] != -1:
            continue
        if chi == parent:
            continue
        used[chi] = 0
        cnt += 1
        dfs(fr,chi,cur)
    return

for i in range(N):
    cnt = 1
    if used[i] != -1:
        continue
    used[i] = 0
    dfs(fr,i,-1)
    MAX = max(MAX, cnt)
print(MAX)

Buggy submission (verdict: Runtime Error):
import sys
input=sys.stdin.readline
sys.setrecursionlimit(10 ** 8)
from itertools import accumulate
from itertools import permutations
from itertools import combinations
from collections import defaultdict
from collections import Counter
import fractions
import math
from collections import deque
from bisect import bisect_left
from bisect import bisect_right
from bisect import insort_left
import itertools
from heapq import heapify
from heapq import heappop
from heapq import heappush
import heapq
from copy import deepcopy
from decimal import Decimal
alf = list("abcdefghijklmnopqrstuvwxyz")
ALF = list("ABCDEFGHIJKLMNOPQRSTUVWXYZ")
#import numpy as np
INF = float("inf")
#d = defaultdict(int)
#d = defaultdict(list)
N,M = map(int,input().split())
count = [[0]*N for _ in range(N)]
fr = [[] for _ in range(N)]
for _ in range(M):
    a,b = map(int,input().split())
    a,b = a-1,b-1
    if count[a][b] == 1:
        continue
    count[a][b] = 1
    count[b][a] = 1
    fr[a].append(b)
    fr[b].append(a)
used = [-1]*N 
MAX = 0
def dfs(fr,cur,parent,cnt):
    global MAX
    MAX = max(cnt,MAX)
    children = fr[cur]
    for chi in children:
        if chi == parent or used[chi] != -1:
            continue
        used[chi] = 0
        cnt += 1
        dfs(fr,chi,cur,cnt)
    return

for i in range(N):
    cnt = 1
    if used[i] != -1:
        continue
    used[i] = 0
    dfs(fr,i,-1,cnt)
print(MAX)