Java排序算法三之归并排序的递归与非递归的案例分析

package mergesort;

import java.util.Arrays;
import java.util.Random;
import java.util.Scanner;
//归并排序的非递归算法
public class MergeSort{
  public static void main(String args[]){
    MergeSort mer = new MergeSort();
    int[] array = mer.getArray();
    System.out.println("OriginalArray:" + Arrays.toString(array));
    mer.mergeSort(array);
    System.out.println("SortedArray:" + Arrays.toString(array));
  }
  public int[] getArray(){
    Scanner cin = new Scanner(System.in);
    System.out.print("Input the length of Array:");
    int length = cin.nextInt();
    int[] arr = new int[length];
    Random r = new Random();
    for(int i = 0; i < length; i++){
      arr[i] = r.nextInt(100);
    }
    cin.close();
    return arr;
  }
  public void mergeSort(int[] a){
    int len = 1;
    while(len < a.length){
      for(int i = 0; i < a.length; i += 2*len){
        merge(a, i, len);
      }
      len *= 2;
    }
  }

  public void merge(int[] a, int i, int len){
    int start = i;
    int len_i = i + len;//归并的前半部分数组
    int j = i + len;
    int len_j = j +len;//归并的后半部分数组
    int[] temp = new int[2*len];
    int count = 0;
    while(i < len_i && j < len_j && j < a.length){
      if(a[i] <= a[j]){
        temp[count++] = a[i++];
      }
      else{
        temp[count++] = a[j++];
      }
    }
    while(i < len_i && i < a.length){//注意：这里i也有可能超过数组长度
      temp[count++] = a[i++];
    }
    while(j < len_j && j < a.length){
      temp[count++] = a[j++];
    }
    count = 0;
    while(start < j && start < a.length){
      a[start++] = temp[count++];
    }
  }
}

package mergesort;

public class MergeSort {
  public static void mergeSort(int[] data,int left,int right){ //left,right均为数字元素下标
    if(left<right){
      int half=(left+right)/2;
      mergeSort(data,left,half);
      mergeSort(data,half+1,right);
      merge(data,left,right);
    }
  }
  public static void merge(int []a,int l,int h){
    int mid=(l+h)/2;
    int i=l;
    int j=mid+1;
    int count=0;
    int temp[]=new int[h-l+1];
    while(i<=mid&&j<=h){
      if(a[i]<a[j]){
        temp[count++]=a[i++];
      }else{
        temp[count++]=a[j++];
      }    
    }
    while(i<=mid){
      temp[count++]=a[i++];
    }
    while(j<=h){
      temp[count++]=a[j++];
    }
    count=0;
    while(l<=h){
      a[l++]=temp[count++];
    }
  }
  public static void printArray(int arr[]){
    for(int k=0;k<arr.length;k++){
      System.out.print(arr[k]+"\t");
    }
  }
  public static int[] getArray(){
//   int[] data={4,2,3,1};
    int[] data={543,23,45,65,76,1,456,7,77,88,3,9};
    return data;
  }

  public static void main(String args[]){
    int[]a=getArray();
    System.out.print("数组排序前：");
    printArray(a);
    System.out.print("\n");
    mergeSort(a,0,a.length-1);
    System.out.print("归并排序后：");
    printArray(a);
  }
}