148. Sort List

148. Sort List

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

/** * Definition for singly-linked list. * public class ListNode { * int val; * ListNode next; * ListNode(int x) { val = x; } * } */class Solution { private ListNode findMiddle(ListNode head) { ListNode slow = head, fast = head.next; while (fast != null && fast.next != null) { fast = fast.next.next; slow = slow.next; } return slow; } private ListNode merge(ListNode head1, ListNode head2) { ListNode dummy = new ListNode(0); ListNode tail = dummy; while (head1 != null && head2 != null) { if (head1.val < head2.val) { tail.next = head1; head1 = head1.next; } else { tail.next = head2; head2 = head2.next; } tail = tail.next; } if (head1 != null) { tail.next = head1; } else { tail.next = head2; } return dummy.next; } public ListNode sortList(ListNode head) { if (head == null || head.next == null) { return head; } ListNode mid = findMiddle(head); ListNode right = sortList(mid.next); mid.next = null; ListNode left = sortList(head); return merge(left, right); }}

/** * Definition for singly-linked list. * function ListNode(val, next) { * this.val = (val===undefined ? 0 : val) * this.next = (next===undefined ? null : next) * } *//** * @param {ListNode} head * @return {ListNode} */var sortList = function(head) { if (!head || !head.next) return head; let stash = []; while (head) { stash.push(head); const temp = head.next; head.next = null; head = temp; } stash.sort((a, b) => a.val - b.val).reduce((pre, cur) => (pre.next = cur)); return stash[0];};

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