Problem:
Score : 200 points

Problem Statement

There are N rabbits, numbered 1 through N.

The i-th (1≤i≤N) rabbit likes rabbit a_i.
Note that no rabbit can like itself, that is, a_i≠i.

For a pair of rabbits i and j (i＜j), we call the pair (i，j) a friendly pair if the following condition is met.

Rabbit i likes rabbit j and rabbit j likes rabbit i.

Calculate the number of the friendly pairs.

Constraints

2≤N≤10^5

1≤a_i≤N

a_i≠i

Input

The input is given from Standard Input in the following format:

N
a_1 a_2 ... a_N

Output

Print the number of the friendly pairs.

Sample Input 1

4
2 1 4 3

Sample Output 1

2

There are two friendly pairs: (1，2) and (3，4).

Sample Input 2

3
2 3 1

Sample Output 2

0

There are no friendly pairs.

Sample Input 3

5
5 5 5 5 1

Sample Output 3

1

There is one friendly pair: (1，5).

Solution in Java7 (OpenJDK 1.7.0) (judge verdict: Accepted)


import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.NoSuchElementException;

/**
 * Created by jaga on 9/22/16.
 */
public class Main {
    public static void solve(PrintWriter out, FastScanner in) {
        int N = in.nextInt();
        int a[] = new int[N + 1];
        int res = 0;
        for(int i = 1 ; i <= N ; i ++){
            int now  = in.nextInt();
            a[i] = now;
            if(a[now] == i){
                res ++;
                a[i] = now;
            }else{
                a[i] = now;
            }
        }
        System.out.println(res);
    }

    /*以下はテンプレ*/
    public static void main(String args[]) {
        OutputStream outputStream = System.out;
        PrintWriter out = new PrintWriter(outputStream);
        FastScanner in = new FastScanner();
        solve(out, in);
        out.close();
    }

    public static class FastScanner {
        private final InputStream in = System.in;
        private final byte[] buffer = new byte[1024];
        private int ptr = 0;
        private int bufferLength = 0;

        private boolean hasNextByte() {
            if (ptr < bufferLength) {
                return true;
            } else {
                ptr = 0;
                try {
                    bufferLength = in.read(buffer);
                } catch (IOException e) {
                    e.printStackTrace();
                }
                if (bufferLength <= 0) {
                    return false;
                }
            }
            return true;
        }

        private int readByte() {
            if (hasNextByte()) return buffer[ptr++];
            else return -1;
        }

        private static boolean isPrintableChar(int c) {
            return 33 <= c && c <= 126;
        }

        private void skipUnprintable() {
            while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
        }

        boolean hasNext() {
            skipUnprintable();
            return hasNextByte();
        }

        public String next() {
            if (!hasNext()) throw new NoSuchElementException();
            StringBuilder sb = new StringBuilder();
            int b = readByte();
            while (isPrintableChar(b)) {
                sb.appendCodePoint(b);
                b = readByte();
            }
            return sb.toString();
        }

        long nextLong() {
            if (!hasNext()) throw new NoSuchElementException();
            long n = 0;
            boolean minus = false;
            int b = readByte();
            if (b == '-') {
                minus = true;
                b = readByte();
            }
            if (b < '0' || '9' < b) {
                throw new NumberFormatException();
            }
            while (true) {
                if ('0' <= b && b <= '9') {
                    n *= 10;
                    n += b - '0';
                } else if (b == -1 || !isPrintableChar(b)) {
                    return minus ? -n : n;
                } else {
                    throw new NumberFormatException();
                }
                b = readByte();
            }
        }

        double nextDouble() {
            return Double.parseDouble(next());
        }

        double[] nextDoubleArray(int n) {
            double[] array = new double[n];
            for (int i = 0; i < n; i++) {
                array[i] = nextDouble();
            }
            return array;
        }

        double[][] nextDoubleMap(int n, int m) {
            double[][] map = new double[n][];
            for (int i = 0; i < n; i++) {
                map[i] = nextDoubleArray(m);
            }
            return map;
        }

        public int nextInt() {
            return (int) nextLong();
        }

        public int[] nextIntArray(int n) {
            int[] array = new int[n];
            for (int i = 0; i < n; i++) array[i] = nextInt();
            return array;
        }

        public long[] nextLongArray(int n) {
            long[] array = new long[n];
            for (int i = 0; i < n; i++) array[i] = nextLong();
            return array;
        }

        public String[] nextStringArray(int n) {
            String[] array = new String[n];
            for (int i = 0; i < n; i++) array[i] = next();
            return array;
        }

        public char[][] nextCharMap(int n) {
            char[][] array = new char[n][];
            for (int i = 0; i < n; i++) array[i] = next().toCharArray();
            return array;
        }

        public int[][] nextIntMap(int n, int m) {
            int[][] map = new int[n][];
            for (int i = 0; i < n; i++) {
                map[i] = nextIntArray(m);
            }
            return map;
        }
    }
}