private ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if (l1 == null) return l2;
if (l2 == null) return l1;
ListNode c1 = l1.val <= l2.val ? l1 : l2;
ListNode c2 = l1.val <= l2.val ? l2 : l1;
while (c1.next != null && c2 != null) {
if (c1.val <= c2.val && c2.val <= c1.next.val) {
ListNode tmp = c2.next;
c2.next = c1.next;
c1.next = c2;
c1 = c1.next;
c2 = tmp;
} else c1 = c1.next;
}
if (c1.next == null) c1.next = c2;
return l1.val <= l2.val ? l1 : l2;
}
/**
* merge sort
*
* @param head
* @return
*/
public ListNode sortList(ListNode head) {
if (head == null || head.next == null) return head;
ListNode slow = head, fast = head.next;
while (fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
}
/* separate the list into two lists. */
ListNode newHead = slow.next;
slow.next = null;
/* sort two lists. */
head = sortList(head);
newHead = sortList(newHead);
/* combine two lists. */
return mergeTwoLists(head, newHead);
}