Sort_list——Sort a linked list in O(n log n) time using constant space complexity. 合并排序

最新推荐文章于 2023-10-09 02:02:27 发布

原创最新推荐文章于 2023-10-09 02:02:27 发布 · 413 阅读

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本文介绍了一种使用归并排序思想实现的链表排序算法，该算法可以在O(nlogn)的时间复杂度内完成排序，并且仅使用常数级别的额外空间。通过递归地将链表分为两半，分别排序后再进行合并，最终实现整个链表的有序排列。

Sort a linked list in O(n log n) time using constant space complexity.

/* class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) {
*         val = x;
*         next = null;
*     }
* }
* */
public class Sort_list {
public ListNode sortList(ListNode head) {
  if(head == null||head.next == null){
   return head;
  }

  ListNode mid = getMid(head);
  ListNode midnext = mid.next;
  mid.next = null;//将链表分成两个部分，head-fast;slow-链表最后节点
  return mergeSort(sortList(head), sortList(midnext));//先分后并
}

public ListNode getMid(ListNode head){
  if(head == null||head.next==null){
   return head;
  }
  ListNode fast = head;
  ListNode slow = head;
  while(fast!=null&&fast.next!=null&&fast.next.next!=null){
   fast = fast.next.next;
   slow = slow.next;
  }
  return slow;
}

public ListNode mergeSort(ListNode n1,ListNode n2){
  ListNode preHead = new ListNode(0);//新建一个头节点
  ListNode cur1 = n1;
  ListNode cur2 = n2;
  ListNode cur = preHead;
  while (cur1 != null && cur2 != null) {
   if (cur1.val < cur2.val) {
    cur.next = cur1;
    cur1 = cur1.next;
   }else {
    cur.next = cur2;
    cur2 = cur2.next;
   }
   cur = cur.next;
  }
   cur.next = cur1 == null?cur2:cur1;
   return preHead.next;
}
}