Leetcode(Java)-148. 排序链表

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;
    }
}