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:
输入: 4->2->1->3
输出: 1->2->3->4
示例 2:
输入: -1->5->3->4->0
输出: -1->0->3->4->5
思路:归并排序
/**
* 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;
ListNode sentinel = new ListNode(0);
sentinel.next = head;
ListNode slow = sentinel;
ListNode fast = sentinel;
while(fast!=null && fast.next!=null)
{
slow = slow.next;
fast = fast.next.next;
}
ListNode temp = slow.next;
slow.next = null;
ListNode left = sortList(head);
ListNode right = sortList(temp);
ListNode h = new ListNode(0);
ListNode res = h;
while(left!=null && right!=null)
{
if(left.val < right.val)
{
h.next = left;
left = left.next;
}
else
{
h.next = right;
right = right.next;
}
h = h.next;
}
h.next = left == null ? right : left;
return res.next;
}
}