Submission #23613523

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Source Code Expand

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())

n = int(input())
a = list(map(int, input().split()))

b = []

change = dict()
cnt = 0
for v in a:
    if v in change:
        b += [change[v]]
    else:
        b += [cnt]
        change[v] = cnt
        cnt += 1

#m = (n + 1) // 2

kaibun = UnionFind(n)

for i in range(n):
    kaibun.union(b[i], b[n - i - 1])

res = 0

for g in kaibun.all_group_members().values():
    res += len(g) - 1

print(res)

Submission Info

Judge Result