148. Sort List
Sort a linked list in O(n log n) time using constant space complexity.
merge sort
public class Solution { public ListNode sortList(ListNode head) { if (head == null || head.next == null) return head; // step 1. cut the list to two halves ListNode prev = null, slow = head, fast = head; while (fast != null && fast.next != null) { prev = slow; slow = slow.next; fast = fast.next.next; } prev.next = null; // step 2. sort each half ListNode l1 = sortList(head); ListNode l2 = sortList(slow); // step 3. merge l1 and l2 return merge(l1, l2); } ListNode merge(ListNode l1, ListNode l2) { ListNode l = new ListNode(0), p = l; 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 l.next; } }