2181. Merge Nodes in Between Zeros

/**
 * 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; }
 * }
 */
public class Solution {

    public ListNode mergeNodes(ListNode head) {
        // Initialize a sentinel/dummy node with the first non-zero value.
        ListNode current = head.next;
        ListNode nextSum = current;

        while (nextSum != null) {
            int sum = 0;
            // Find the sum of all nodes until you encounter a 0.
            while (nextSum.val != 0) {
                sum += nextSum.val;
                nextSum = nextSum.next;
            }

            // Assign the sum to the current node's value.
            current.val = sum;
            // Move nextSum to the first non-zero value of the next block.
            nextSum = nextSum.next;
            // Move current also to this node.
            current.next = nextSum;
            current = current.next;
        }
        return head.next;
    }
}