图论

借用图片 https://www.acwing.com/solution/content/7917/

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import java.util.*;

public class Main {
    static final int N = 10010;
    static int[] b = new int[N];
    static boolean[] st = new boolean[N];
    static int n;

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        n = sc.nextInt();

        for (int i = 1; i <= n; i++) b[i] = sc.nextInt();

        int cnt = 0;
        //类似邻接表的实现
        for (int i = 1; i <= n; i++)
            if (!st[i]) {
                cnt++;
                for (int j = i; !st[j]; j = b[j]) 
                    st[j] = true;
            }

        System.out.println(n - cnt);
    }
}