148. Sort List
https://leetcode.com/problems/sort-list/
Sort a linked list in O(n log n) time using constant space complexity.
Example 1:
Input: 4->2->1->3 Output: 1->2->3->4
Example 2:
Input: -1->5->3->4->0 Output: -1->0->3->4->5---
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| /** | |
| * Definition for singly-linked list. | |
| * public class ListNode { | |
| * int val; | |
| * ListNode next; | |
| * ListNode() {} | |
| * ListNode(int val) { this.val = val; } | |
| * ListNode(int val, ListNode next) { this.val = val; this.next = next; } | |
| * } | |
| */ | |
| class Solution { | |
| public ListNode sortList(ListNode head) { | |
| // Base case => terminate | |
| if (head == null || head.next == null) { | |
| return head; | |
| } | |
| ListNode prev = null; | |
| ListNode slow = head; | |
| ListNode fast = head; | |
| while (fast != null && fast.next != null) { | |
| prev = slow; | |
| slow = slow.next; | |
| fast = fast.next.next; | |
| } | |
| prev.next = null; | |
| ListNode l1 = sortList(head); | |
| ListNode l2 = sortList(slow); | |
| return merge(l1, l2); | |
| } | |
| private ListNode merge(ListNode l1, ListNode l2) { | |
| ListNode curr = new ListNode(-1); | |
| ListNode ans = curr; | |
| while (l1 != null && l2 != null) { | |
| if (l1.val < l2.val) { | |
| curr.next = l1; | |
| l1 = l1.next; | |
| } else { | |
| curr.next = l2; | |
| l2 = l2.next; | |
| } | |
| curr = curr.next; | |
| } | |
| if (l1 != null) { | |
| curr.next = l1; | |
| } | |
| if (l2 != null) { | |
| curr.next = l2; | |
| } | |
| return ans.next; | |
| } | |
| } |
