Joey LIU | NANTSOU


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/**
 * 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 pivot = split(head);
        ListNode second = pivot.next;
        pivot.next = null;
        return merge(sortList(head), sortList(second));
    }
    
    private ListNode split(ListNode head) {
        ListNode slow = head, fast = head;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }
    
    private ListNode merge(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode node = head;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                node.next = l1;
                l1 = l1.next;
            } else {
                node.next = l2;
                l2 = l2.next;
            }
            node = node.next;
        }
        if (l1 != null) {
            node.next = l1;
        }
        if (l2 != null) {
            node.next = l2;
        }
        return head.next;
    }
}