LeetCode: 148. Sort List

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148. Sort List

LeetCode - 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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/**
 * Runtime: 4 ms, faster than 62.99% of Java online submissions for Sort List.
 *
 * Memory Usage: 40.2 MB, less than 89.47% of Java online submissions for Sort List.
 */
public ListNode sortList(ListNode head) {
    if (Objects.isNull(head) || Objects.isNull(head.next)) {
        return head;
    }
    ListNode prev = null;
    ListNode slow = head;
    ListNode fast = head;
    while (Objects.nonNull(fast) && Objects.nonNull(fast.next)) {
        prev = slow;
        slow = slow.next;
        fast = fast.next.next;
    }
    prev.next = null;

    ListNode firstResult = sortList(head);
    ListNode secondResult = sortList(slow);

    return merge(firstResult, secondResult);
}

private ListNode merge(ListNode first, ListNode second) {
    ListNode result = new ListNode(0);
    ListNode tail = result;
    while (Objects.nonNull(first) && Objects.nonNull(second)) {
        if (first.val < second.val) {
            tail.next = first;
            first = first.next;
        } else {
            tail.next = second;
            second = second.next;
        }
        tail = tail.next;
    }
    if (Objects.isNull(first)) {
        tail.next = second;
    }
    if (Objects.isNull(second)) {
        tail.next = first;
    }
    return result.next;
}