黄忠的技术博客

插入排序

• 标签：
• tec

我们来看看Collections.sort(List list)排序方法的实现，可以总结如下：

1. 使用Arrays.sort(T[])来排序
2. 最终(排序数组的大小小于7)使用插入排序
3. 使用二分的方式拆分需要排序的数组，直到数组的大小小于7
4. 使用递归的方式
5. 合并被拆分后被排序的数组

Collections.java 关键代码如下

public static <T extends Comparable<? super T>> void sort(List<T> list) { Object[] a = list.toArray(); Arrays.sort(a); ListIterator<T> i = list.listIterator(); for (int j=0; j<a.length; j++) { i.next(); i.set((T)a[j]); } }

Arrays 关键代码如下

```
public static void sort(Object[] a) { Object[] aux = (Object[])a.clone(); mergeSort(aux, a, 0, a.length, 0); }

/**
 * Src is the source array that starts at index 0
 * Dest is the (possibly larger) array destination with a possible offset
 * low is the index in dest to start sorting
 * high is the end index in dest to end sorting
 * off is the offset to generate corresponding low, high in src
 */
private static void mergeSort(Object[] src,
			  Object[] dest,
			  int low,
			  int high,
			  int off) {
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 &&
		 ((Comparable) dest[j-1]).compareTo(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);
    mergeSort(dest, src, mid, high, -off);

    // 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 (((Comparable)src[mid-1]).compareTo(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 && ((Comparable)src[p]).compareTo(src[q])<=0)
            dest[i] = src[p++];
        else
            dest[i] = src[q++];
    }
} ```