Algs4-2.2.20间接排序

2.2.20间接排序。编写一个不改变数组的归并排序，它返回一个int[]数组perm,其中perm[i]的值是原数组中第i小的元素的位置。

public class E2d2d20
{
    private static int subArrayLenTrunONInsertionSort=15;
    public static int[] sort(Comparable[] a)
    {
        int len=a.length;
        int[] perm=new int[len];
        int[] aux=new int[len];
        for(int i=0;i<len;i++)
        {
            perm[i]=i;
            aux[i]=i;
        }
        sort(perm,aux,a,0,a.length-1);
        return perm;
   }
    private static void sort(int[] perm,int[] aux,Comparable[] a,int lo,int hi)
    {
        if ((hi-lo+1)<=subArrayLenTrunONInsertionSort)
        {
          insertionSort(perm,a,lo,hi);
        }
        else
        {
          int mid=lo+(hi-lo)/2;
          sort(aux,perm,a,lo,mid);
          sort(aux,perm,a,mid+1,hi);
       
          if(!less(a[aux[mid+1]],a[aux[mid]]))
          {
              for(int i=lo;i<=hi;i++)
                  perm[i]=aux[i];
              return;
          }
         
          merge(aux,perm,a,lo,mid,hi);
        }
    }

   
    private static void merge(int[] perm,int[] aux,Comparable[] a,int lo,int mid,int hi)
    {
      int i=lo;
      int j=mid+1;
      for(int k=lo;k<=hi;k++)
      {
           if (i>mid) aux[k]=perm[j++];
           else if (j>hi) aux[k]=perm[i++];
           else if (less(a[perm[j]],a[perm[i]])) aux[k]=perm[j++];
           else                           aux[k]=perm[i++];
      }
      }
    private static boolean less(Comparable v,Comparable w)
    { return v.compareTo(w)<0;}
   
    private static void exch(int[] perm,int i,int j)
    {
      int t=perm[i];
      perm[i]=perm[j];
      perm[j]=t;
    }
   
    private static void insertionSort(int[] perm,Comparable[] a,int lo,int hi)
    {
      for (int i=lo+1;i<=hi;i++)
        for (int j=i;j>lo && less(a[perm[j]],a[perm[j-1]]);j--)
            exch(perm,j,j-1);
    }
   
 public static boolean isSorted(int[] perm,Comparable[] a)
    {
      for(int i=1;i<a.length;i++)
        if(less(a[perm[i]],a[perm[i-1]])) return false;
      return true;
    }
 
 public static void main(String[] args)
 {
     Integer N=Integer.parseInt(args[0]);
     Comparable[] a=new Comparable[N];
     for(int i=0;i<N;i++)
         a[i]=StdRandom.uniform();
     int[] perm=sort(a);
     StdOut.println("Sorted="+isSorted(perm,a));
 }
}