/******************************************************************************
 *  Compilation:  javac UF.java
 *  Execution:    java UF < input.txt
 *  Dependencies: StdIn.java StdOut.java
 *  Data files:   https://algs4.cs.princeton.edu/15uf/tinyUF.txt
 *                https://algs4.cs.princeton.edu/15uf/mediumUF.txt
 *                https://algs4.cs.princeton.edu/15uf/largeUF.txt
 *
 *  Weighted quick-union by rank with path compression by halving.
 *
 *  % java UF < tinyUF.txt
 *  4 3
 *  3 8
 *  6 5
 *  9 4
 *  2 1
 *  5 0
 *  7 2
 *  6 1
 *  2 components
 *
 ******************************************************************************/


/**
 *  The {@code UF} class represents a <em>union–find data type</em>
 *  (also known as the <em>disjoint-sets data type</em>).
 *  It supports the classic <em>union</em> and <em>find</em> operations,
 *  along with a <em>count</em> operation that returns the total number
 *  of sets.
 *  <p>
 *  The union–find data type models a collection of sets containing
 *  <em>n</em> elements, with each element in exactly one set.
 *  The elements are named 0 through <em>n</em>–1.
 *  Initially, there are <em>n</em> sets, with each element in its
 *  own set. The <em>canonical element</em> of a set
 *  (also known as the <em>root</em>, <em>identifier</em>,
 *  <em>leader</em>, or <em>set representative</em>)
 *  is one distinguished element in the set. Here is a summary of
 *  the operations:
 *  <ul>
 *  <li><em>find</em>(<em>p</em>) returns the canonical element
 *      of the set containing <em>p</em>. The <em>find</em> operation
 *      returns the same value for two elements if and only if
 *      they are in the same set.
 *  <li><em>union</em>(<em>p</em>, <em>q</em>) merges the set
 *      containing element <em>p</em> with the set containing
 *      element <em>q</em>. That is, if <em>p</em> and <em>q</em>
 *      are in different sets, replace these two sets
 *      with a new set that is the union of the two.
 *  <li><em>count</em>() returns the number of sets.
 *  </ul>
 *  <p>
 *  The canonical element of a set can change only when the set
 *  itself changes during a call to <em>union</em>&mdash;it cannot
 *  change during a call to either <em>find</em> or <em>count</em>.
 *  <p>
 *  This implementation uses <em>weighted quick union by rank</em>
 *  with <em>path compression by halving</em>.
 *  The constructor takes &Theta;(<em>n</em>) time, where
 *  <em>n</em> is the number of elements.
 *  The <em>union</em> and <em>find</em> operations take
 *  &Theta;(log <em>n</em>) time in the worst case.
 *  The <em>count</em> operation takes &Theta;(1) time.
 *  Moreover, starting from an empty data structure with <em>n</em> sites,
 *  any intermixed sequence of <em>m</em> <em>union</em> and <em>find</em>
 *  operations takes <em>O</em>(<em>m</em> &alpha;(<em>n</em>)) time,
 *  where &alpha;(<em>n</em>) is the inverse of
 *  <a href = "https://en.wikipedia.org/wiki/Ackermann_function#Inverse">Ackermann's function</a>.
 *  <p>
 *  For alternative implementations of the same API, see
 *  {@link QuickUnionUF}, {@link QuickFindUF}, and {@link WeightedQuickUnionUF}.
 *  For additional documentation, see
 *  <a href="https://algs4.cs.princeton.edu/15uf">Section 1.5</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */

public class UF {

    private int[] parent;  // parent[i] = parent of i
    private byte[] rank;   // rank[i] = rank of subtree rooted at i (never more than 31)
    private int count;     // number of components

    /**
     * Initializes an empty union-find data structure with
     * {@code n} elements {@code 0} through {@code n-1}.
     * Initially, each element is in its own set.
     *
     * @param  n the number of elements
     * @throws IllegalArgumentException if {@code n < 0}
     */
    public UF(int n) {
        if (n < 0) throw new IllegalArgumentException();
        count = n;
        parent = new int[n];
        rank = new byte[n];
        for (int i = 0; i < n; i++) {
            parent[i] = i;
            rank[i] = 0;
        }
    }

    /**
     * Returns the canonical element of the set containing element {@code p}.
     *
     * @param  p an element
     * @return the canonical element of the set containing {@code p}
     * @throws IllegalArgumentException unless {@code 0 <= p < n}
     */
    public int find(int p) {
        validate(p);
        while (p != parent[p]) {
            parent[p] = parent[parent[p]];    // path compression by halving
            p = parent[p];
        }
        return p;
    }

    /**
     * Returns the number of sets.
     *
     * @return the number of sets (between {@code 1} and {@code n})
     */
    public int count() {
        return count;
    }

    /**
     * Returns true if the two elements are in the same set.
     *
     * @param  p one element
     * @param  q the other element
     * @return {@code true} if {@code p} and {@code q} are in the same set;
     *         {@code false} otherwise
     * @throws IllegalArgumentException unless
     *         both {@code 0 <= p < n} and {@code 0 <= q < n}
     * @deprecated Replace with two calls to {@link #find(int)}.
     */
    @Deprecated
    public boolean connected(int p, int q) {
        return find(p) == find(q);
    }

    /**
     * Merges the set containing element {@code p} with the set
     * containing element {@code q}.
     *
     * @param  p one element
     * @param  q the other element
     * @throws IllegalArgumentException unless
     *         both {@code 0 <= p < n} and {@code 0 <= q < n}
     */
    public void union(int p, int q) {
        int rootP = find(p);
        int rootQ = find(q);
        if (rootP == rootQ) return;

        // make root of smaller rank point to root of larger rank
        if      (rank[rootP] < rank[rootQ]) parent[rootP] = rootQ;
        else if (rank[rootP] > rank[rootQ]) parent[rootQ] = rootP;
        else {
            parent[rootQ] = rootP;
            rank[rootP]++;
        }
        count--;
    }

    // validate that p is a valid index
    private void validate(int p) {
        int n = parent.length;
        if (p < 0 || p >= n) {
            throw new IllegalArgumentException("index " + p + " is not between 0 and " + (n-1));
        }
    }

    /**
     * Reads an integer {@code n} and a sequence of pairs of integers
     * (between {@code 0} and {@code n-1}) from standard input, where each integer
     * in the pair represents some element;
     * if the elements are in different sets, merge the two sets
     * and print the pair to standard output.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        int n = StdIn.readInt();
        UF uf = new UF(n);
        while (!StdIn.isEmpty()) {
            int p = StdIn.readInt();
            int q = StdIn.readInt();
            if (uf.find(p) == uf.find(q)) continue;
            uf.union(p, q);
            StdOut.println(p + " " + q);
        }
        StdOut.println(uf.count() + " components");
    }
}