Algs4-2.2.10快速归并
2.2.10快速归并。实现一个merge()方法,按降序将a[]的后半部分复制到aux[],然后将其归并回a[]中。这样就可以去掉内循环中检测某半边是否用尽的代码。注意:这样的排序产生的结果是不稳定的(请见2.5.1.8节)。
public class Merge2
{
private static Comparable[] aux;
public static int ArrayAccessTimes=0;
public static void sort(Comparable[] a)
{
aux=new Comparable[a.length];
sort(a,0,a.length-1);
}
public static void sort(Comparable[] a,int lo,int hi)
{
if (hi<=lo) return;
int mid=lo+(hi-lo)/2;
sort(a,lo,mid);
sort(a,mid+1,hi);
ArrayAccessTimes++;
merge(a,lo,mid,hi);
}
public static void merge(Comparable[] a,int lo,int mid,int hi)
{
int i=lo,j=hi;
for (int k=lo;k<=mid;k++)
aux[k]=a[k];
/*
//k 索引更容易理解的方式
for (int k=0;mid+1+k<=hi;k++)
aux[mid+1+k]=a[hi-k];
*/
for (int k=mid+1;k<=hi;k++)
aux[k]=a[hi-k+mid+1];
for(int k=lo;k<=hi;k++)
if (less(aux[j],aux[i])) a[k]=aux[j--];
else a[k]=aux[i++];
}
private static boolean less(Comparable v,Comparable w)
{ return v.compareTo(w)<0;}
public static boolean isSorted(Comparable[] a)
{
for(int i=1;i<a.length;i++)
if(less(i,i-1)) return false;
return true;
}
public static void main(String[] args)
{
int N=Integer.parseInt(args[0]);
Double[] a=new Double[N];
for(int i=0;i<N;i++)
a[i]=StdRandom.uniform();
sort(a);
StdOut.printf("isSorted=%s",isSorted(a));
}
}
public class Merge2
{
private static Comparable[] aux;
public static int ArrayAccessTimes=0;
public static void sort(Comparable[] a)
{
aux=new Comparable[a.length];
sort(a,0,a.length-1);
}
public static void sort(Comparable[] a,int lo,int hi)
{
if (hi<=lo) return;
int mid=lo+(hi-lo)/2;
sort(a,lo,mid);
sort(a,mid+1,hi);
ArrayAccessTimes++;
merge(a,lo,mid,hi);
}
public static void merge(Comparable[] a,int lo,int mid,int hi)
{
int i=lo,j=hi;
for (int k=lo;k<=mid;k++)
aux[k]=a[k];
/*
//k 索引更容易理解的方式
for (int k=0;mid+1+k<=hi;k++)
aux[mid+1+k]=a[hi-k];
*/
for (int k=mid+1;k<=hi;k++)
aux[k]=a[hi-k+mid+1];
for(int k=lo;k<=hi;k++)
if (less(aux[j],aux[i])) a[k]=aux[j--];
else a[k]=aux[i++];
}
private static boolean less(Comparable v,Comparable w)
{ return v.compareTo(w)<0;}
public static boolean isSorted(Comparable[] a)
{
for(int i=1;i<a.length;i++)
if(less(i,i-1)) return false;
return true;
}
public static void main(String[] args)
{
int N=Integer.parseInt(args[0]);
Double[] a=new Double[N];
for(int i=0;i<N;i++)
a[i]=StdRandom.uniform();
sort(a);
StdOut.printf("isSorted=%s",isSorted(a));
}
}