算法（Algorithms）第4版 练习 1.5.22

package com.qiusongde;

import edu.princeton.cs.algs4.StdOut;
import edu.princeton.cs.algs4.StdStats;

public class Exercise1522 {

    public static double timeTrialForQF(int T, int N, int[] edges) {

        Stopwatch timer  = new Stopwatch();

        // repeat the experiment T times
        for (int t = 0; t < T; t++) {
            edges[t] = ErdosRenyi.countByQF(N);
        }

        return timer.elapsedTime();

    }

    public static double timeTrialForWeiQU(int T, int N, int[] edges) {

        Stopwatch timer  = new Stopwatch();

        // repeat the experiment T times
        for (int t = 0; t < T; t++) {
            edges[t] = ErdosRenyi.countByWeiQU(N);
        }

        return timer.elapsedTime();

    }

    public static double mean(int[] edges) {
        return StdStats.mean(edges);
    }

    public static void main(String[] args) {

        int T = Integer.parseInt(args[0]);

        int edgesQF[] = new int[T];
        int edgesQU[] = new int[T];

        double prevQF = timeTrialForQF(T, 125, edgesQF);
        double prevQU = timeTrialForWeiQU(T, 125, edgesQU);

        for(int N = 250; true; N += N) {

            double timeQF = timeTrialForQF(T, N, edgesQF);
            double timeQU = timeTrialForWeiQU(T, N, edgesQU);

            double meanQFConnect = mean(edgesQF);
            double meanQUconnect = mean(edgesQU);

            StdOut.printf("%6d %7.1f %7.1f %7.1f %7.1f\n", N, meanQFConnect, timeQF/prevQF, meanQUconnect, timeQU/prevQU);

            prevQF = timeQF;
            prevQU = timeQU;
        }

    }

}

package com.qiusongde;

import edu.princeton.cs.algs4.StdOut;
import edu.princeton.cs.algs4.StdRandom;
import edu.princeton.cs.algs4.StdStats;
import edu.princeton.cs.algs4.UF;
import edu.princeton.cs.algs4.WeightedQuickUnionUF;

public class ErdosRenyi {

    public static int countByUF(int N) {

        int edges = 0;
        UF uf = new UF(N);

        while (uf.count() > 1) {
            int i = StdRandom.uniform(N);
            int j = StdRandom.uniform(N);
            uf.union(i, j);
            edges++;
        }

        return edges;

    }

    public static int countByQF(int N) {

        int edges = 0;
        UFQuickFind uf = new UFQuickFind(N);

        while (uf.count() > 1) {
            int i = StdRandom.uniform(N);
            int j = StdRandom.uniform(N);
            uf.union(i, j);
            edges++;
        }

        return edges;

    }

    public static int countByQU(int N) {

        int edges = 0;
        UFQuickUnion uf = new UFQuickUnion(N);

        while (uf.count() > 1) {
            int i = StdRandom.uniform(N);
            int j = StdRandom.uniform(N);
            uf.union(i, j);
            edges++;
        }

        return edges;

    }

    public static int countByWeiQU(int N) {

        int edges = 0;
        WeightedQuickUnionUF uf = new WeightedQuickUnionUF(N);

        while (uf.count() > 1) {
            int i = StdRandom.uniform(N);
            int j = StdRandom.uniform(N);
            uf.union(i, j);
            edges++;
        }

        return edges;

    }

    public static void main(String[] args) {

             int n = Integer.parseInt(args[0]);          // number of vertices
            int trials = Integer.parseInt(args[1]);     // number of trials
            int[] edges = new int[trials];

            // repeat the experiment trials times
            for (int t = 0; t < trials; t++) {
                edges[t] = countByUF(n);
            }

            // report statistics
            StdOut.println("1/2 n ln n = " + 0.5 * n * Math.log(n));
            StdOut.println("mean       = " + StdStats.mean(edges));
            StdOut.println("stddev     = " + StdStats.stddev(edges));
    }

}

package com.qiusongde;

public class Stopwatch {

    private final long start;

    public Stopwatch() {
        start = System.currentTimeMillis();
    }

    public double elapsedTime() {
        long now = System.currentTimeMillis();
        return (now - start) / 1000.0;
    }

}

结果：

   250   761.1     2.9   761.4     1.0
   500  1698.6     2.8  1717.8     5.2
  1000  3739.0     3.8  3737.7     2.0
  2000  8184.8     3.7  8174.3     2.1
  4000 17773.4     3.9 17799.1     2.2
  8000 38312.6     4.1 38229.6     2.2