Quick-find

1 2/******************************************************************************3 *  Compilation:  javac QuickFindUF.java4 *  Execution:  java QuickFindUF < input.txt5 *  Dependencies: StdIn.java StdOut.java6 *  Data files:   https://algs4.cs.princeton.edu/15uf/tinyUF.txt7 *                https://algs4.cs.princeton.edu/15uf/mediumUF.txt8 *                https://algs4.cs.princeton.edu/15uf/largeUF.txt9 *10 *  Quick-find algorithm.11 *12 ******************************************************************************/13 14import java.util.Arrays;15import java.io.*;16 17 18public class QuickFindUF {19    private int[] id;    // id[i] = component identifier of i20    private int count;   // number of connect components21 22    /*23     * Initializes an empty union-find data structure with24     */25 26    public QuickFindUF(int n) {27        count = n;28        id = new int[n];29        for (int i = 0; i < n; i++)30            id[i] = i;31    }32 33    /*34     * Returns the number of sets.35     */36    public int count() {37        return count;38    }39  40    /**41     * Returns the canonical element of the set containing element {@code p}.42     */43    public int find(int p) {44        validate(p);45        return id[p];46    }47 48    // validate that p is a valid index49    private void validate(int p) {50        int n = id.length;51        if (p < 0 || p >= n) {52            throw new IllegalArgumentException("index " + p + " is not between 0 and " + (n-1));53        }54    }55 56    /**57     * Returns true if the two elements are in the same set.58     * 59     * @param  p one element60     * @param  q the other element61     * @return {@code true} if {@code p} and {@code q} are in the same set;62     *         {@code false} otherwise63     * @throws IllegalArgumentException unless64     *         both {@code 0 <= p < n} and {@code 0 <= q < n}65     * @deprecated Replace with two calls to {@link #find(int)}.66     */67    @Deprecated68    public boolean connected(int p, int q) {69        validate(p);70        validate(q);71        return id[p] == id[q];72    }73  74    /**75     * Merges the set containing element {@code p} with the 76     * the set containing element {@code q}.77     *78     * @param  p one element79     * @param  q the other element80 81     */82    public void union(int p, int q) {83        validate(p);84        validate(q);85        int pID = id[p];   // needed for correctness86        int qID = id[q];   // to reduce the number of array accesses87 88        // p and q are already in the same component89        if (pID == qID) return;90 91        for (int i = 0; i < id.length; i++)92            if (id[i] == pID) id[i] = qID;93        count--;94    }95 96    /**97     * Reads an integer {@code n} and a sequence of pairs of integers98     * (between {@code 0} and {@code n-1}) from standard input, where each integer99     * in the pair represents some element;100     * if the elements are in different sets, merge the two sets101     * and print the pair to standard output.102     */103    public static void main(String[] args) {104        int n = StdIn.readInt();105        QuickFindUF uf = new QuickFindUF(n);106        while (!StdIn.isEmpty()) {107            int p = StdIn.readInt();108            int q = StdIn.readInt();109            if (uf.find(p) == uf.find(q)) continue;110            uf.union(p, q);111            StdOut.println(p + " " + q);112        }113        StdOut.println(uf.count() + " components");114    }115 116}117 118