在 O(n log n) 时间复杂度和常数级空间复杂度下，对链表进行排序。
示例 1:
代码语言：javascript复制输入: 4->2->1->3
输出: 1->2->3->4示例 2:
代码语言：javascript复制输入: -1->5->3->4->0
输出: -1->0->3->4->5解：归并
代码语言：javascript复制/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
class Solution {
public ListNode sortList(ListNode head) {
if (head == null || head.next == null) {
return head;
}
// 快慢指针找中点
ListNode prev = null;
ListNode slow = head;
ListNode fast = head;
while (fast != null && fast.next != null) {
prev = slow;
slow = slow.next;
fast = fast.next.next;
}
prev.next = null;
//排序
ListNode l1 = sortList(head);
ListNode l2 = sortList(slow);
//合并
return merge(l1, l2);
}
private ListNode merge(ListNode l1, ListNode l2) {
ListNode l = new ListNode(0);
ListNode p = l;
while (l1 != null && l2 != null) {
if (l1.val < l2.val) {
p.next = l1;
l1 = l1.next;
} else {
p.next = l2;
l2 = l2.next;
}
p = p.next;
}
if (l1 != null)
p.next = l1;
if (l2 != null)
p.next = l2;
return l.next;
}
}
