奶牛洗牌(The Bovine Shuffle S) 题解

思路分析

按题目要求使用对应数据结构与算法模拟/递推求解，注意数据范围边界与时间复杂度。

参考代码

#include <iostream>
#include <vector>
using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n;
    if (!(cin >> n)) return 0;
    vector<int> a(n + 1), indeg(n + 1, 0);
    for (int i = 1; i <= n; ++i) {
        cin >> a[i];
        indeg[a[i]]++;
    }
    vector<int> q;
    q.reserve(n);
    for (int i = 1; i <= n; ++i) {
        if (indeg[i] == 0) q.push_back(i);
    }
    for (size_t head = 0; head < q.size(); ++head) {
        int u = q[head];
        int v = a[u];
        if (--indeg[v] == 0) q.push_back(v);
    }
    cout << (n - (int)q.size()) << "\n";
    return 0;
}