Buggy python submission (verdict: Runtime Error):
# union-find
# https://note.nkmk.me/python-union-find/

class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n
        # 各要素の親要素を格納する

    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 x > y:
        if self.parents[x] > self.parents[y]:
            self.parents[x] += self.parents[y]
            self.parents[y] = x
            return
        self.parents[y] += self.parents[x]   
        self.parents[x] = y
        return
    
    def size(self, x):
        x = self.find(x)    
        return self.parents[x]*(-1)

def main():
    n, m = map(int, input().split())
    friend = UnionFind(n)

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

if  __name__ == "__main__":
     main()


Fixed submission:
# union-find
# https://note.nkmk.me/python-union-find/

class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n
        # 各要素の親要素を格納する

    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            return self.find(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
        return
    
    def size(self, x):
        x = self.find(x)    
        return self.parents[x]*(-1)

def main():
    n, m = map(int, input().split())
    friend = UnionFind(n)

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

if  __name__ == "__main__":
     main()

Buggy python submission (verdict: Runtime Error):
# union-find
# https://note.nkmk.me/python-union-find/

class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n
        # 各要素の親要素を格納する

    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 x > y:
            self.parents[x] += self.parents[y]
            self.parents[y] = x
            return
        self.parents[y] += self.parents[x]   
        self.parents[x] = y
        return
    
    def size(self, x):
        x = self.find(x)    
        return self.parents[x]*(-1)

def main():
    n, m = map(int, input().split())
    friend = UnionFind(n)

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

if  __name__ == "__main__":
     main()


Fixed submission:
# union-find
# https://note.nkmk.me/python-union-find/

class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n
        # 各要素の親要素を格納する

    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            return self.find(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
        return
    
    def size(self, x):
        x = self.find(x)    
        return self.parents[x]*(-1)

def main():
    n, m = map(int, input().split())
    friend = UnionFind(n)

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

if  __name__ == "__main__":
     main()