_148.java

package com.fishercoder.solutions;

import com.fishercoder.common.classes.ListNode;

/**
 * 148. Sort List
 *
 * Sort a linked list in O(n log n) time using constant space complexity.
 */
public class _148 {

    /**Credit: https://discuss.leetcode.com/topic/18100/java-merge-sort-solution
     * But this is not using constant space.*/
    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) return head;

        //Step 1: split the list into halves
        ListNode prev = null;
        ListNode slow = head;
        ListNode fast = head;
        while (fast != null && fast.next != null) {
            prev = slow;
            fast = fast.next.next;
            slow = slow.next;
        }
        prev.next = null;

        //step 2: sort each half
        ListNode l1 = sortList(head);
        ListNode l2 = sortList(slow);

        //step 3: merge the two halves
        return merge(l1, l2);
    }

    private ListNode merge(ListNode l1, ListNode l2) {
        ListNode result = new ListNode(0);
        ListNode tmp = result;

        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                tmp.next = l1;
                l1 = l1.next;
            } else {
                tmp.next = l2;
                l2 = l2.next;
            }
            tmp = tmp.next;
        }

        if (l1 != null) {
            tmp.next = l1;
        }
        if (l2 != null) {
            tmp.next = l2;
        }
        return result.next;
    }

}