希尔排序

希尔排序(Shell Sort) 实质上是插入排序(Insertion Sort)的变种，它以一组序列为间隔进行插入排序，其具体代码如下：

import edu.princeton.cs.algs4.StdOut;
public class Shell {

     public static void sort(Comparable[] a) {
         int h = 1;
         while (h < a.length/3) {  h = h*3 + 1; }
         while (h >= 1) {
            for (int i = h; i < a.length; i++) {
                for (int j = i; j >= h; j = j - h) {
                    if (less(a[j - h],a[j]))                j = 0;
                    else                        exch(a, j - h, j);
                }               
            }
            h = h/3;
         }
        }

        private static boolean less(Comparable v, Comparable w) {
            if (v.compareTo(w) <= 0) return true;
            else                     return false;
        }

        private static void exch(Comparable[] a, int i, int j) {
            Comparable temp = a[i];
                       a[i] = a[j];
                       a[j] = temp;
        }

        private static  void show(Comparable[] a) {
            for (Comparable item : a) {
                StdOut.print(item + "  ");
            }
            StdOut.println();
        }

        private static boolean isSorted(Comparable[] a) {
            for (int i = 0; i < a.length - 1; i++) {
                if (!less(a[i], a[i+1])) return false;
            }
            return true;
        }
        public static void main(String[] args) {
            // TODO Auto-generated method stub
            Comparable[] a = {2, 3, 4, 1, 7, 8, 6, 5, 0, -10};
            Shell.sort(a);
            assert isSorted(a);
        }
}

希尔排序的复杂度目前没有统一的说法，但一般认为它的复杂度低于平方级别。