148-排序链表.md

##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

/**
 * 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)  //空链表或者只有单个结点
            return head;
        
        //使用快慢指针寻找中间 结点
         ListNode slow = head;
         ListNode fast = head;
         while (fast.next!=null && fast.next.next !=null){
             slow = slow.next;
             fast = fast.next.next;
         }


        ListNode ptr1 = slow.next;
        slow.next = null;

        ListNode start1 = sortList(head);
        ListNode start2 = sortList(ptr1);
        
        if (start1 == null) return start2;

        if (start2 == null) return start1;

        ListNode header = new ListNode(-1);
        ListNode pre = header;

        while (start1 != null && start2 != null) {
            if (start1.val <= start2.val) {
                pre.next = start1;
                start1 = start1.next;
                pre = pre.next;
            } else {
                pre.next = start2;
                start2 = start2.next;
                pre = pre.next;
            }
        }

        if (start1 == null) {
            pre.next = start2;
        } else {
            pre.next = start1;
        }

        return header.next;

    }
}