封裝成類：
package com.roc.algorithms.sort;
/**
* 歸并排序
*
* @author imroc
*/
public class MergeSort {
//自頂向下方式
public static void sortUpToDown(int[] a) {
int[] aux = new int[a.length];
sort(a, aux, 0, a.length - 1);
}
//自底向上方式
public static void sortDownToUp(int[] a) {
int[] aux = new int[a.length];
for (int sz = 1; sz < a.length; sz *= 2) {
for (int lo = 0; lo < a.length - sz; lo += 2 * sz) {
merge(a, aux, lo, lo + sz - 1, Math.min(lo + 2 * sz - 1, a.length - 1));
}
}
}
private static void merge(int[] a, int[] aux, int lo, int mid, int hi) {
for (int k = lo; k <= hi; k++) {
aux[k] = a[k];
}
int i = lo, j = mid + 1;
for (int k = lo; k <= hi; k++) {
if (i > mid) {
a[k] = aux[j++];
} else if (j > hi) {
a[k] = aux[i++];
} else if (aux[j] < aux[i]) {
a[k] = aux[j++];
} else {
a[k] = aux[i++];
}
}
}
private static void sort(int[] a, int[] aux, int lo, int hi) {
if (lo >= hi) {
return;
}
int mid = (lo + hi) >> 1;
sort(a, aux, lo, mid);
sort(a, aux, mid + 1, hi);
merge(a, aux, lo, mid, hi);
}
}
測試：
int[] a = {9, 0, 6, 5, 8, 2, 1, 7, 4, 3};
System.out.println(Arrays.toString(a));
MergeSort.sortUpToDown(a);
//MergeSort.sortDownToUp(a);
System.out.println(Arrays.toString(a));
輸出：
[9, 0, 6, 5, 8, 2, 1, 7, 4, 3]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
