WeightedQuickUnionByHeightUF.java


Below is the syntax highlighted version of WeightedQuickUnionByHeightUF.java from §1.5 Case Study: Union-Find.


/******************************************************************************
 *  Compilation:  javac WeightedQuickUnionByHeightUF.java
 *  Execution:    java  WeightedQuickUnionByHeightUF < 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 height (instead of by size).
 * 
 ******************************************************************************/

/**
 *  The {@code WeightedQuickUnionByHeightUF} class represents a union–find data structure.
 *  It supports the <em>union</em> and <em>find</em> operations, along with
 *  methods for determining whether two sites are in the same component
 *  and the total number of components.
 *  <p>
 *  This implementation uses weighted quick union by height (without path compression).
 *  Initializing a data structure with <em>n</em> sites takes linear time.
 *  Afterwards, <em>union</em>, <em>find</em>, and <em>connected</em> take
 *  logarithmic time (in the worst case) and <em>count</em> takes constant
 *  time.
 *  <p>
 *  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 WeightedQuickUnionByHeightUF {
    private int[] parent;   // parent[i] = parent of i
    private int[] height;   // height[i] = height of subtree rooted at i
    private int count;  // number of components

    /**
     * Initializes an empty union–find data structure with {@code n} sites
     * {@code 0} through {@code n-1}. Each site is initially in its own 
     * component.
     *
     * @param  n the number of sites
     * @throws IllegalArgumentException if {@code n < 0}
     */
    public WeightedQuickUnionByHeightUF(int n) {
        count = n;
        parent = new int[n];
        height = new int[n];
        for (int i = 0; i < n; i++) {
            parent[i] = i;
            height[i] = 0;
        }
    }

    /**
     * Returns the number of components.
     *
     * @return the number of components (between {@code 1} and {@code n})
     */
    public int count() {
        return count;
    }
  
    /**
     * Returns the component identifier for the component containing site {@code p}.
     *
     * @param  p the integer representing one site
     * @return the component identifier for the component containing site {@code p}
     * @throws IllegalArgumentException unless {@code 0 <= p < n}
     */
    public int find(int p) {
        validate(p);
        while (p != parent[p])
            p = parent[p];
        return p;
    }

    // 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));
        }
    }

    /**
     * Returns true if the the two sites are in the same component.
     *
     * @param  p the integer representing one site
     * @param  q the integer representing the other site
     * @return {@code true} if the two sites {@code p} and {@code q} are in the same component;
     *         {@code false} otherwise
     * @throws IllegalArgumentException unless
     *         both {@code 0 <= p < n} and {@code 0 <= q < n}
     */
    public boolean connected(int p, int q) {
        return find(p) == find(q);
    }

    /**
     * Merges the component containing site {@code p} with the 
     * the component containing site {@code q}.
     *
     * @param  p the integer representing one site
     * @param  q the integer representing the other site
     * @throws IllegalArgumentException unless
     *         both {@code 0 <= p < n} and {@code 0 <= q < n}
     */
    public void union(int p, int q) {
        int i = find(p);
        int j = find(q);
        if (i == j)
            return;

        // make shorter root point to taller one
        if      (height[i] < height[j]) parent[i] = j;
        else if (height[i] > height[j]) parent[j] = i;
        else {
            parent[j] = i;
            height[i]++;
        }
        count--;
    }

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


Copyright © 2000–2017, Robert Sedgewick and Kevin Wayne.
Last updated: Fri Oct 20 12:50:46 EDT 2017.