归并排序 求逆序数 链表的归并排序 多线程归并排序 java

import java.util.Scanner;


public class Main {
    private static int count=0;
    public static void mergesort(int a[],int low,int high)
    {
        if(low<high)
        {
            int mid=(low+high)>>1;
             mergesort(a,low,mid);
             mergesort(a,mid+1,high);
             merge(a,low,mid,high);
            
            
            
        }
        
        
    }

    private static void merge(int[] a, int low, int mid, int high) {
        
        int temp[]=new int[high-low+1]; //开辟额外空间 
        int index=0;
        int beg1=low;
        int beg2=mid+1;
        while(beg1<=mid&&beg2<=high)  //如果两者都存在，选在较少的一个
        {
            if(a[beg1]<=a[beg2])
            {
                temp[index++]=a[beg1++];
            }
            else
            {
                temp[index++]=a[beg2++];
                count+=mid-beg1+1;    //   如果选中后者，对于后者会出现mid-beg1+1的逆序数
            }
            
        }
        while(beg1<=mid)  //剩下的元素
        {
            temp[index++]=a[beg1++];
        }
        while(beg2<=high)
        {
            temp[index++]=a[beg2++];
        }
        for(int i=0;i<index;i++)
        {
            a[low+i]=temp[i];
        }
        
        
        
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner scn=new Scanner(System.in);
        int len=scn.nextInt();
        while(len-->0)
        {
            count=0;
            int len2=scn.nextInt();
            int a[]=new int[len2];
            for(int i=0;i<len2;i++)
            {
                a[i]=scn.nextInt();
            }
            
            mergesort(a,0,len2-1);
            
            
            System.out.println(count);
        }
        

    }

}

 leetcode 中的链表排序

public class Solution {
    public ListNode sortList(ListNode head) {
        
        if(head==null) return null;
        if(head.next==null) return head;
          ListNode list=split(head);
          return merge(sortList(head),sortList(list));
        
        
    }
  //divide into two part ,return the middle address
    private ListNode split(ListNode head) {
        
        ListNode qucik=head;
        ListNode slow=head;
        ListNode pre=null;
        
        while(qucik!=null)
        {
           pre=slow;
           slow=slow.next;
           qucik=qucik.next;
           if(qucik!=null) qucik=qucik.next;
            
            
        }
        
        pre.next=null;
        return slow;
        
        
    }
    public ListNode merge(ListNode head,ListNode middle)
    {
        ListNode p1=head;
        ListNode p2=middle;
        ListNode h=new ListNode(-1);
        ListNode tail=h; //insert into tail;
        
        while(p1!=null&&p2!=null)
        {
            if(p1.val<=p2.val)
            {
                tail.next=p1;
                tail=tail.next;
                p1=p1.next;
                
            }
            else
            {
                tail.next=p2;
                tail=tail.next ;
                p2=p2.next;
                
            }
            
            
            
        }
     if(p1!=null)
     {
        
         tail.next=p1;
         tail=tail.next ;
        
     }
     if(p2!=null)
     {
         tail.next=p2;
         tail=tail.next ;
         
     }
        
        
        
        return h.next;
    }
    
    
    
    
}

多线程 的归并排序