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
时间： 2024-10-16 14:13:26