2816. Double a Number Represented as a Linked List

Problem

You are given the head of a non-empty linked list representing a non-negative integer without leading zeroes.

Return **the *head* of the linked list after doubling it**.

Example 1:

Input: head = [1,8,9]
Output: [3,7,8]
Explanation: The figure above corresponds to the given linked list which represents the number 189. Hence, the returned linked list represents the number 189 * 2 = 378.

Example 2:

Input: head = [9,9,9]
Output: [1,9,9,8]
Explanation: The figure above corresponds to the given linked list which represents the number 999. Hence, the returned linked list reprersents the number 999 * 2 = 1998.

Constraints:

• The number of nodes in the list is in the range [1, 104]
• 0 <= Node.val <= 9
• The input is generated such that the list represents a number that does not have leading zeros, except the number 0 itself.

Solution (Java)

/**
 * 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 doubleIt(ListNode head) {
        if (head.val > 4)
            head = new ListNode(0, head);
        for(ListNode node = head; node != null; node = node.next) {
            node.val = (node.val * 2) % 10;
            if (node.next != null && node.next.val > 4)
                node.val++;
        }
        return head;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).