Sort List

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

Solution

/**
 * 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;

        if (head.next.next == null) {
            ListNode one = head;
            ListNode two = head.next;

            if (one.val <= two.val) {
                return head;
            } else {

                two.next = one;
                one.next = null;
                return two;

            }


        }

        ListNode slow = head;
        ListNode fast = head;

        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }

        ListNode second = slow.next;
        slow.next = null;

        ListNode fist1 = sortList(head);
        ListNode second1 = sortList(second);

        return merge(fist1, second1);
    }

    public ListNode merge(ListNode m, ListNode n) {
        ListNode dummy = new ListNode(0);

        ListNode runner = dummy;
        while (m != null && n != null) {
            if (m.val <= n.val) {
                runner.next = m;
                m = m.next;
            } else {
                runner.next = n;
                n = n.next;
            }
            runner = runner.next;
        }

        if (m == null) {
            runner.next = n;
        }

        if (n == null) {
            runner.next = m;
        }

        return dummy.next;
    }
}