SortList148.java

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

/**
 * Definition for singly-linked list.
 * public class ListNode {
 * int val;
 * ListNode next;
 * ListNode(int x) { val = x; }
 * }
 */

public class SortList148 {
 public ListNode sortList(ListNode head) {
 if (head == null) return null;
 if (head.next == null) return head;

 ListNode s = head;
 ListNode f = head;
 ListNode pre = s;
 while (s != null && f != null && f.next != null) {
 pre = s;
 s = s.next;
 f = f.next.next;
 }

 pre.next = null;
 ListNode left = sortList(head);
 ListNode right = sortList(s);
 return merge(left, right);
 }

 private ListNode merge(ListNode left, ListNode right) {
 ListNode res = new ListNode(-1);
 ListNode p = res;
 ListNode l = left;
 ListNode r = right;
 while (l != null && r != null) {
 if (l.val <= r.val) {
 p.next = l;
 l = l.next;
 } else {
 p.next = r;
 r = r.next;
 }
 p = p.next;
 }
 if (l != null) p.next = l;
 if (r != null) p.next = r;
 return res.next;
 }


}