Sort a linked list in O(n log n) time using constant space complexity.
Total Accepted: 70033 Total Submissions: 281580 Difficulty: Medium

Tags
Linked List Sort
Similar Problems
(E) Merge Two Sorted Lists (M) Sort Colors (M) Insertion 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) {
        // ref : https://leetcode.com/discuss/44369/java-merge-sort-solution
        // test case: [1, 3, 5,  4, 2]
        if (head == null || head.next == null)  {
            return head;
        }
        // step1. 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;
        
        // step2 : sort each half
        ListNode l1 = sortList(head);
        ListNode l2 = sortList(slow);
        
        // step3 : merge l1 and l2
       return merge(l1, l2);
    }
    
    public 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;
    }
}