2816. Double a Number Represented as a Linked List

Medium

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:

/**
 * Definition for a singly-linked list.
 * class ListNode {
 *     public $val = 0;
 *     public $next = null;
 *     function __construct($val = 0, $next = null) {
 *         $this->val = $val;
 *         $this->next = $next;
 *     }
 * }
 */
class Solution {

    /**
     * @param ListNode $head
     * @return ListNode
     */
    function doubleIt($head) {
        if ($head->val >= 5)
            $head = new ListNode(0, $head);

        for ($curr = $head; $curr != null; $curr = $curr->next) {
            $curr->val *= 2;
            $curr->val %= 10;
            if ($curr->next && $curr->next->val >= 5)
                ++$curr->val;
        }

        return $head;
    }
}

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