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;
}
}
