Sort a linked list in O(n log n) time using constant space complexity.
/**
* Definition for singly-linked list.
* class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
public ListNode sortList(ListNode head) {
if(head == null || head.next == null) {
return head;
}
ListNode mid = getMid(head);
ListNode midNext = mid.next;
mid.next = null;
return mergeSort(sortList(head), sortList(midNext));
}
private ListNode getMid(ListNode head) {
if(head == null || head.next == null) {
return head;
}
ListNode slow = head, quick = head;
while(quick.next != null && quick.next.next != null) {
slow = slow.next;
quick = quick.next.next;
}
return slow;
}
private ListNode mergeSort(ListNode n1, ListNode n2) {
ListNode preHead = new ListNode(0);
ListNode cur1 = n1;
ListNode cur2 = n2;
ListNode cur = preHead;
while(cur1 != null && cur2 != null) {
if(cur1.val < cur2.val) {
cur.next = cur1;
cur1 = cur1.next;
} else {
cur.next = cur2;
cur2 = cur2.next;
}
cur = cur.next;
}
if(cur1 != null){
cur.next = cur1;
}
if(cur2 != null){
cur.next = cur2;
}
return preHead.next;
}
}