链表的排序

题目描述

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

由于限制了时间复杂度，所以选择使用归并排序

/** * Definition for singly-linked list. * class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { *         val = x; *         next = null; *     } * } */public class Solution {    public ListNode sortList(ListNode head) {        if(head == null || head.next==null)            return head;                ListNode slow = head;        ListNode fast = head;        while(fast.next!=null && fast.next.next!=null){            slow = slow.next;            fast = fast.next.next;        }                fast = slow.next;        slow.next = null;        slow =  sortList(head);        fast = sortList(fast);        head = mergeSort(slow,fast);          return head;    }        public ListNode mergeSort(ListNode head1,ListNode head2){        if(head1==null)            return head2;        if(head2 ==null)            return head1;        ListNode head=null;                if(head1.val>head2.val){            head = head2;            head.next = mergeSort(head1,head2.next);        }else{            head = head1;            head.next = mergeSort(head1.next,head2);        }                return head;    }}