148. Sort List

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

含义：为一个列表排序，要求空间复杂度为常量

思路：使用归并排序

class Solution {
 private ListNode mergeNode(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode p = head;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                p.next = l1;
                l1 = l1.next;
            } else {
                p.next = l2;
                l2 = l2.next;
            }
            p = p.next;
        }
        if (l1 != null) p.next = l1;
        if (l2 != null) p.next = l2;
        return head.next;
    }
    
    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode walker = head, runner = head;
        while (runner != null && runner.next != null) {
            walker = walker.next;
            runner = runner.next.next;
        }
        ListNode l1 = sortList(head);
        ListNode l2 = sortList(walker);
        return mergeNode(l1, l2);        
    }
}