Sort a linked list inO(nlogn) time using constant space complexity.

注意和数组类的merge sort的区别，linkedlist先把list分成两半

tag: linkedlist

/**
 * 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 slow = head, fast = head, prev = null;
        while (fast != null && fast.next != null){
            prev = slow;
            slow = slow.next;
            fast = fast.next.next;
        }
        prev.next = null;

        ListNode l1 = sortList(head);
        ListNode l2 = sortList(slow);

        return merge(l1, l2);
    }

    private ListNode merge(ListNode l1, ListNode l2){
        ListNode dummy = new ListNode(-1);
        ListNode cur = dummy;

        while (l1 != null && l2 != null){
            if (l1.val < l2.val){
                cur.next = l1;
                l1 = l1.next;
            }
            else{
                cur.next = l2;
                l2 = l2.next;
            }
            cur = cur.next;
        }

        if (l1 != null){
            cur.next = l1;
        }

        if (l2 != null){
            cur.next = l2;
        }

        return dummy.next;
    }
}