java 实现归并排序, 自顶向下

Merge.java

import java.util.Date;

public class Merge {
  private static Comparable[] aux;

  public static void sort(Comparable[] a) {
    aux = new Comparable[a.length];
    sort(a, 0, a.length - 1);
  }

  private 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);

    // 如果 a[mid] 小于等于 a[mid + 1], 则可以认为数组已经是有序的了, 可以跳过 merge()
    if (less(a[mid], a[mid + 1])) {
      return;
    }

    merge(a, lo, mid, hi);
  }

  public static void merge(Comparable[] a, int lo, int mid, int hi) {
    int i = lo; int j = mid + 1;

    for (int k = lo; k <= hi; k++) {
      aux[k] = a[k];
    }

    for (int k = lo; k <= hi; k++) {
      if (i > mid) {
        a[k] = aux[j++];
      } else if (j > hi) {
        a[k] = aux[i++];
      } else if (less(aux[j], aux[i])) {
        a[k] = aux[j++];
      } else {
        a[k] = aux[i++];
      }
    }

  }


  // v 是否比 w 小 ? 如果返回 true: v < w
  private static boolean less(Comparable v, Comparable w) {
    return v.compareTo(w) < 0;
  }

  private static void exch(Comparable[] a, int i, int j) {
    Comparable t = a[i];
    a[i] = a[j];
    a[j] = t;
  }

  private static void show(Comparable[] a) {
    for (int i = 0; i < a.length; i++) {
      System.out.print(a[i] + " ");
    }
    System.out.println();
  }

  public static boolean isSorted(Comparable[] a) {
    for (int i = 1; i < a.length; i++) {
      if (less(a[i], a[i - 1])) {
        return false;
      }
    }
    return true;
  }

  public static long timeRandomInput(int len, int count) {
    long total = 0;
    Comparable[] a;
    for (int i = 0; i < count; i++) {
      a = Utils.generateRandomIntArray(len, 0, len - 1);
      long start = (new Date()).getTime();
      sort(a);
      total += ((new Date()).getTime()) - start;
    }
    return total;
  }

  public static void main(String[] args) {
    int len = 1000;
    int min = 10;
    int max = 1000;

//    int minIndex = 0;
//    int maxIndex = len - 1;
//    int midIndex = minIndex + (maxIndex - minIndex) / 2;
    Comparable[] a = Utils.generateRandomIntArray(len, min, max);
//    show(a);
    sort(a);
    assert isSorted(a);

//    aux = new Comparable[a.length];
//    merge(a, minIndex, midIndex, maxIndex);
//    show(a);
  }
}