0148. Sort List

/**
 * 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;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode l2 = sortList(slow.next);
        slow.next = null;
        ListNode l1 = sortList(head);
        return mergeList(l1, l2);
    }

    public ListNode mergeList(ListNode l1, ListNode l2) {
        ListNode resPre = new ListNode(-1), curr = resPre;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                curr.next = l1;
                l1 = l1.next;
            }
            else {
                curr.next = l2;
                l2 = l2.next;
            }
            curr = curr.next;
        }
        if (l1 == null) {
            curr.next = l2;
        }
        else {
            curr.next = l1;
        }
        return resPre.next;
    }
}