算法（Algorithms）第4版 练习 2.1.24

代码实现：

package com.qiusongde;

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

public class InsertionWithSentinel {

    public static void sort(Comparable[] a) {

        int N = a.length;

        for(int i = N - 1; i > 0; i--) {
            if(less(a[i], a[i-1])) {
                exch(a, i, i-1);
//                show(a);
            }
        }

        for(int i = 2; i < N; i++) {
            for(int j = i; less(a[j], a[j-1]); j--) {
                exch(a, j, j-1);
            }
//            show(a);
        }

    }

    private static boolean less(Comparable v, Comparable w) {

        return v.compareTo(w) < 0;

    }

    private static void exch(Comparable[] a, int i, int j) {

        Comparable t = a[i];
        a[i] = a[j];
        a[j] = t;

    }

    private static void show(Comparable[] a) {

        //print the array, on a single line.
        for(int i = 0; i < a.length; i++) {
            StdOut.print(a[i] + " ");
        }
        StdOut.println();

    }

    public static boolean isSorted(Comparable[] a) {

        for(int i = 1; i < a.length; i++) {
            if(less(a[i], a[i-1]))
                return false;
        }

        return true;

    }

    public static void main(String[] args) {
        //Read strings from standard input, sort them, and print.
        String[] a = In.readStrings();
        show(a);
        sort(a);
        assert isSorted(a);
        show(a);
    }

}

SortCompare：

package com.qiusongde;

import edu.princeton.cs.algs4.Shell;
import edu.princeton.cs.algs4.StdOut;
import edu.princeton.cs.algs4.StdRandom;

public class SortCompare {

    public static double timeRandomInput(String alg, int N, int T) {

        double total = 0.0;

        Double[] a = new Double[N];
        for(int t = 0; t < T; t++) {
            for(int i = 0; i < N; i++)
                a[i] = StdRandom.uniform();
            total += time(alg, a);
        }

        return total;

    }

    public static double time(String alg, Double[] a) {

        Stopwatch timer = new Stopwatch();

        if(alg.equals("Selection"))
            Selection.sort(a);
        if(alg.equals("Insertion"))
            Insertion.sort(a);
        if(alg.equals("InsertionHalfExchange"))
            InsertionHalfExchange.sort(a);
        if(alg.equals("InsertionWithSentinel"))
            InsertionWithSentinel.sort(a);
        if(alg.equals("Shell"))
            Shell.sort(a);

        return timer.elapsedTime();

    }

    public static void main(String[] args) {

        String alg1 = "Insertion";
        String alg2 = "InsertionWithSentinel";
        int N = 2000;
        int T = 1000;

        double t1 = timeRandomInput(alg1, N, T);
        double t2 = timeRandomInput(alg2, N, T);

        StdOut.printf("For %d random Doubles %d trials\n", N, T, alg1);
        StdOut.printf("%s is %.1fs %s is %.1fs", alg1, t1, alg2, t2);

    }

}

测试结果：

For 2000 random Doubles 1000 trials
Insertion is 4.5s InsertionWithSentinel is 3.7s