/*
 * @lc app=leetcode id=148 lang=java
 *
 * [148] Sort List
 */

// @lc code=start
/**
 * 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, fast = head, prev = null;
        while(fast != null && fast.next != null) {
            fast = fast.next.next;
            prev = slow;
            slow = slow.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(0), tail = dummy, tmp;
        dummy.next = null;
        while(l1 != null || l2 != null) {
            if(l1 == null || l2 == null) {
                if(l1 != null) {
                    tail.next = l1;
                } else {
                    tail.next = l2;
                }
                break;
            } else {
                if(l1.val <= l2.val) {
                    tmp = l1;
                    l1 = l1.next;
                } else {
                    tmp = l2;
                    l2 = l2.next;
                }
            }
            tail.next = tmp;
            tail = tail.next;
            tail.next = null;
        }
        return dummy.next;
    }
}
// @lc code=end