[Java] 图解Java排序算法之归并排序

package sortdemo;
import java.util.Arrays;
/**
* Created by chengxiao on 2016/12/8.
*/
public class MergeSort {
  public static void main(String []args){
    int []arr = {9,8,7,6,5,4,3,2,1};
    sort(arr);
    System.out.println(Arrays.toString(arr));
  }
  public static void sort(int []arr){
    int []temp = new int[arr.length];//在排序前，先建好一个长度等于原数组长度的临时数组，避免递归中频繁开辟空间
    sort(arr,0,arr.length-1,temp);
  }
  private static void sort(int[] arr,int left,int right,int []temp){
    if(left<right){
      int mid = (left+right)/2;
      sort(arr,left,mid,temp);//左边归并排序，使得左子序列有序
      sort(arr,mid+1,right,temp);//右边归并排序，使得右子序列有序
      merge(arr,left,mid,right,temp);//将两个有序子数组合并操作
    }
  }
  private static void merge(int[] arr,int left,int mid,int right,int[] temp){
    int i = left;//左序列指针
    int j = mid+1;//右序列指针
    int t = 0;//临时数组指针
    while (i<=mid && j<=right){
      if(arr[i]<=arr[j]){
        temp[t++] = arr[i++];
      }else {
        temp[t++] = arr[j++];
      }
    }
    while(i<=mid){//将左边剩余元素填充进temp中
      temp[t++] = arr[i++];
    }
    while(j<=right){//将右序列剩余元素填充进temp中
      temp[t++] = arr[j++];
    }
    t = 0;
    //将temp中的元素全部拷贝到原数组中
    while(left <= right){
      arr[left++] = temp[t++];
    }
  }
}

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