148. Sort List.md

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

Example 1:

Input: 4->2->1->3
Output: 1->2->3->4

Example 2:

Input: -1->5->3->4->0
Output: -1->0->3->4->5

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode sortList(ListNode head) {
        if(head==null || head.next==null) return head;
        ListNode slow=head;
        ListNode preSlow=null;
        ListNode fast=head;
        while(fast!=null && fast.next!=null){
            preSlow=slow;
            slow=slow.next;
            fast=fast.next.next;
        }
        preSlow.next=null;
        return mergeList(sortList(head),sortList(slow));
    }
    private ListNode mergeList(ListNode l1,ListNode l2){
        ListNode dummy=new ListNode(-1);
        ListNode merge=dummy;
        while(l1!=null && l2!=null){
            if(l1.val<l2.val){
                merge.next=l1;
                l1=l1.next;
                merge=merge.next;
            }
            else{
                merge.next=l2;
                l2=l2.next;
                merge=merge.next;
            }
        }
        merge.next=(l1!=null)?l1:l2;
        return dummy.next;
    }
}