148. Sort List

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode sortList(ListNode head) {
if(head == null || head.next == null) return head;
// get mid
ListNode mid = getmid(head);
//sort
ListNode tmp = mid.next;
mid.next = null;
ListNode midleft = sortList(head);
ListNode midright = sortList(tmp);
//merge
return merge(midleft, midright);
}

public ListNode getmid(ListNode head) {
ListNode fast = head.next;
ListNode slow = head;
while (fast != null && fast.next != null) {
fast = fast.next.next;
slow = slow.next;
}
return slow;
}
public ListNode merge(ListNode midleft, ListNode midright) {
ListNode dummy = new ListNode(0);
ListNode node = dummy;
while (midleft != null && midright != null) {
if (midleft.val < midright.val) {
node.next = midleft;
node = node.next;
midleft = midleft.next;
} else {
node.next = midright;
node = node.next;
midright = midright.next;
}
}
if(midleft != null) node.next = midleft;
if(midright != null) node.next = midright;
return dummy.next;
}
}

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode sortList(ListNode head) {
if(head == null || head.next == null) return head;
//pivot
ListNode pivot = head;
head = head.next;
pivot.next = null;
ListNode p = pivot;
//small, large
ListNode small = new ListNode(0);
ListNode large = new ListNode(0);
ListNode s = small;
ListNode l = large;
while (head != null) {
if (head.val < pivot.val) {
s.next = head;
head = head.next;
s = s.next;
} else if (head.val == pivot.val) {
p.next = head;
head = head.next;
p = p.next;
} else {
l.next = head;
head = head.next;
l = l.next;
}
}

//merge
s.next = null;
l.next = null;
ListNode res = sortList(small.next);
if (res == null) res = pivot;
else {
ListNode tmp = res;
while (tmp.next != null) tmp = tmp.next;
tmp.next = pivot;
}
p.next = sortList(large.next);
return res;
}

}