leetcode(148):Sort List

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

【分析】

要求将一个单链表排序，时间复杂度为O(nlogn)

此题考查单链表的归并排序

1.找到单链表的中间节点,将整个单链表分为两个子链表
2.分别对两个子链表进行归并排序
3.将排好序的子链表合并成一个链表

【Java实现】

public class Solution {    public static ListNode getMiddleOfList(ListNode head) {          ListNode slow = head;          ListNode fast = head.next;          while(fast!=null&&fast.next!=null) {              slow = slow.next;              fast = fast.next.next;          }          return slow;      }      public  ListNode sortList(ListNode head) {        if(head==null || head.next==null) return head;        ListNode mid = getMiddleOfList(head);        ListNode next=mid.next;        mid.next=null;        ListNode l1 = sortList(head);        ListNode l2 = sortList(next);        return mergeList(l1,l2);    }    private static ListNode mergeList(ListNode a, ListNode b) {        ListNode helper = new ListNode(-1);        ListNode cur = helper;        while(a!=null && b!=null){            if(a.val<=b.val){                cur.next = a;                a= a.next;            }else{                cur.next = b;                b= b.next;            }            cur = cur.next;        }        if(a!=null){            cur.next = a;        }        if(b!=null) {            cur.next = b;        }        return helper.next;    }}