算法（Algorithms）第4版 练习 2.1.25

Posted 2020-09-05 我是老邱

代码实现：

package com.qiusongde;

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

public class InsertionHalfExchange {
    
    public static void sort(Comparable[] a) {
        
        int N = a.length;
        
        for(int i = 1; i < N; i++) {
            Comparable temp = a[i];
            int j = i;
            while(j > 0 && less(temp, a[j-1])) {
                a[j] = a[j-1];
                j--;
            }
            a[j] = temp;
//            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);
    }

}

 

单元测试结果：

S O R T E X A M P L E 
O S R T E X A M P L E 
O R S T E X A M P L E 
O R S T E X A M P L E 
E O R S T X A M P L E 
E O R S T X A M P L E 
A E O R S T X M P L E 
A E M O R S T X P L E 
A E M O P R S T X L E 
A E L M O P R S T X E 
A E E L M O P R S T X 
A E E L M O P R S T X 

 

比较Insertion和InsertionHalfExchange 的性能：

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("Shell"))
            Shell.sort(a);
        
        return timer.elapsedTime();
        
    }
    
    public static void main(String[] args) {
        
        String alg1 = "Insertion";
        String alg2 = "InsertionHalfExchange";
        int N = 1000;
        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);
        
    }

}

 

 

For 1000 random Doubles 1000 trials
Insertion is 1.1s InsertionHalfExchange is 0.5s