算法（Algorithms）第4版 练习 2.2.11（2）

关键代码：

    private static void sort(Comparable[] input, int lo, int hi) {
        
        if(lo >= hi)//just one entry in array
            return;
        
        int mid = lo + (hi-lo)/2;
        sort(input, lo, mid);
        sort(input, mid+1, hi);
        
        if(!less(input[mid+1],input[mid]))//input[mid+1] >= input[mid]
            return;
        
        merge(input, lo, mid, hi);
        
    }

整体：

package com.qiusongde;

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

public class MergeSkipMerge {
    
    private static Comparable[] aux;
    
    public static void sort(Comparable[] input) {
        int N = input.length;
        aux = new Comparable[N];
        sort(input, 0, N-1);
    }
    
    private static void sort(Comparable[] input, int lo, int hi) {
        
        if(lo >= hi)//just one entry in array
            return;
        
        int mid = lo + (hi-lo)/2;
        sort(input, lo, mid);
        sort(input, mid+1, hi);
        
        if(!less(input[mid+1],input[mid]))//input[mid+1] >= input[mid]
            return;
        
        merge(input, lo, mid, hi);
        
    }
    
    private static void merge(Comparable[] input, int lo, int mid, int hi) {
        
        //copy input[lo,hi] to aux[lo,hi]
        for(int i = lo; i <= hi; i++) {
            aux[i] = input[i];
        }
        
        int i = lo;
        int j = mid + 1;
        for(int k = lo; k <= hi; k++) {
            if(i > mid)
                input[k] = aux[j++];
            else if(j > hi)
                input[k] = aux[i++];
            else if(less(aux[j], aux[i]))
                input[k] = aux[j++];
            else 
                input[k] = aux[i++];
        }
        
        StdOut.printf("merge(input, %4d, %4d, %4d)", lo, mid, hi);
        show(input);//for test
        
    }
    
    private static boolean less(Comparable v, Comparable w) {
        
        return v.compareTo(w) < 0;
        
    }
    
    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[] input = In.readStrings();
        show(input);//for test
        sort(input);
        assert isSorted(input);
        show(input);//for test
        
    }

}

测试结果：

M E R G E S O R T E X A M P L E 
merge(input,    0,    0,    1)E M R G E S O R T E X A M P L E 
merge(input,    2,    2,    3)E M G R E S O R T E X A M P L E 
merge(input,    0,    1,    3)E G M R E S O R T E X A M P L E 
merge(input,    4,    5,    7)E G M R E O R S T E X A M P L E 
merge(input,    0,    3,    7)E E G M O R R S T E X A M P L E 
merge(input,    8,    8,    9)E E G M O R R S E T X A M P L E 
merge(input,   10,   10,   11)E E G M O R R S E T A X M P L E 
merge(input,    8,    9,   11)E E G M O R R S A E T X M P L E 
merge(input,   14,   14,   15)E E G M O R R S A E T X M P E L 
merge(input,   12,   13,   15)E E G M O R R S A E T X E L M P 
merge(input,    8,   11,   15)E E G M O R R S A E E L M P T X 
merge(input,    0,    7,   15)A E E E E G L M M O P R R S T X 
A E E E E G L M M O P R R S T X 

性能对比：

For 20000 random Doubles 1000 trials
Merge is 3.6s MergeFasterM is 3.3s MergeUseInsert is 3.2s MergeSkipMerge is 3.4s