將數組中的相鄰元素兩兩配對,用歸併算法進行排序,構成n/2組長度爲2的排好序的子數組段,然後再將其排成長度爲4的子數組段,如此繼續下去直到整個數組排好序。
按照此思想,消除遞歸後的歸併排序算法(僞代碼)如下:
public class MergeSort {
public static void mergeSort(Comparable[]a)
{
Comparable[]b=new Comparable[a.length];
int s=1;
while (s<a.length)
{
mergePass(a,b,s);//合併到數組b
s+=s;
mergePass(b,a,s);//合併到數組a
s+=s;
}
}
private static void mergePass(Comparable[] x, Comparable[] y, int s)
{//合併大小爲s的相鄰數組
int i=0;
while (i<=x.length-2*s)
{//合併大小爲s的兩端相鄰字數組
merge(x,y,i,i+s-1,i+s*2-1);
i=i+2*s;
}
//剩下的元素個數小於2s
if (i+s<x.length)
{
merge(x, y, i, i+s-1, x.length-1);
}
else//複製到y
{
for (int j = i; j < x.length; j++)
{
y[j] = x[j];
}
}
}
private static void merge(Comparable[] c, Comparable[] d, int l, int m, int r)
{
//合併c[l:m]和c[m+1,r]到d[l:r]
int i=1, j=m+1,k=l;
while (i<=m && j<=r)
{
if (c[i].compareTo(c[j])<=0)
{
d[k++]=c[i++];
}
else
{
d[k++]=c[j++];
}
}
if (i>m)
{
for (int k2 = 0; k2 <=r; k2++)
{
d[k++] = c[k2];
}
}
else
{
for (int k2 = 0; k2 <=m; k2++)
{
d[k++] = c[k2];
}
}
}
}