改进版的合并排序
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下面是改进版的合并排序,下面是jdk1.7的源码部分
执行步骤如下:
(1)如果比较的长度小于INSERTIONSORT_THRESHOLD插入排序的阈值,直接调用传统的插入排序进行比较
(2)当大于插入排序的阈值时,采用合并排序算法,这里有个改进的地方,红色加亮部分,如果已经排好序的,不再进行比较,而是直接复制过去,提高效率
private static void mergeSort(Object[] src,
Object[] dest,
int low, int high, int off,
Comparator c) {
int length = high - low;
// Insertion sort on smallest arrays
if (length < INSERTIONSORT_THRESHOLD) {
for (int i=low; i<high; i++)
for (int j=i; j>low && c.compare(dest[j-1], dest[j])>0; j--)
swap(dest, j, j-1);
return;
}
// Recursively sort halves of dest into src
int destLow = low;
int destHigh = high;
low += off;
high += off;
int mid = (low + high) >>> 1;
mergeSort(dest, src, low, mid, -off, c);
mergeSort(dest, src, mid, high, -off, c);
//改进地方
// If list is already sorted, just copy from src to dest. This is an
// optimization that results in faster sorts for nearly ordered lists.
if (c.compare(src[mid-1], src[mid]) <= 0) {
System.arraycopy(src, low, dest, destLow, length);
return;
}
// Merge sorted halves (now in src) into dest
for(int i = destLow, p = low, q = mid; i < destHigh; i++) {
if (q >= high || p < mid && c.compare(src[p], src[q]) <= 0)
dest[i] = src[p++];
else
dest[i] = src[q++];
}
}
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