2181. Merge Nodes in Between Zeros

Medium

You are given the head of a linked list, which contains a series of integers separated by 0's. The beginning and end of the linked list will have Node.val == 0.

For every two consecutive 0's, merge all the nodes lying in between them into a single node whose value is the sum of all the merged nodes. The modified list should not contain any 0's.

Return the head of the modified linked list.

Example 1:

Input: head = [0,3,1,0,4,5,2,0]
Output: [4,11]
Explanation: The above figure represents the given linked list. The modified list contains
- The sum of the nodes marked in green: 3 + 1 = 4.
- The sum of the nodes marked in red: 4 + 5 + 2 = 11.

Example 2:

Input: head = [0,1,0,3,0,2,2,0]
Output: [1,3,4]
Explanation: The above figure represents the given linked list. The modified list contains
- The sum of the nodes marked in green: 1 = 1.
- The sum of the nodes marked in red: 3 = 3.
- The sum of the nodes marked in yellow: 2 + 2 = 4.

Constraints:

The number of nodes in the list is in the range [3, 2 * 10⁵].
0 <= Node.val <= 1000
There are no two consecutive nodes with Node.val == 0.
The beginning and end of the linked list have Node.val == 0.

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 $head
     * @return ListNode
     */
    function mergeNodes($head) {
        $dummy = new ListNode(0);
        $current = $dummy;
        $sum = 0;

        // Skip the first zero node
        $head = $head->next;

        while ($head !== null) {
            if ($head->val == 0) {
                $current->next = new ListNode($sum);
                $current = $current->next;
                $sum = 0;
            } else {
                $sum += $head->val;
            }
            $head = $head->next;
        }

        return $dummy->next;
    }
}

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