[LeetCode] Sort List

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

LeetCode: https://oj.leetcode.com/problems/sort-list/ 

题解：本题要求通过常数空间对链表进行排序，可以通过归并排序实现

代码如下：

public class Solution {    public ListNode sortList(ListNode head) {if(head == null || head.next == null)return head;ListNode walker = head;ListNode runner = head;while(runner.next != null && runner.next.next != null){walker = walker.next;runner = runner.next.next;}ListNode head2 = walker.next;walker.next = null;ListNode head1 = sortList(head);head2 = sortList(head2);head = merge(head1, head2);return head;    }public ListNode merge(ListNode head1, ListNode head2) {ListNode head = new ListNode(0);ListNode tail = head;while(head1 != null && head2 != null){if(head1.val < head2.val){tail.next = head1;head1 = head1.next;}else{tail.next = head2;head2 = head2.next;}tail = tail.next;}if(head1 == null){tail.next = head2;}else{tail.next = head1;}return head.next;}}