2181 - Merge Nodes in Between Zeros

 JAVA

class Solution {
    public ListNode mergeNodes(ListNode head) {
        ListNode dummy = new ListNode();
        int s = 0;
        ListNode tail = dummy;
        for (ListNode cur = head.next; cur != null; cur = cur.next) {
            if (cur.val != 0) {
                s += cur.val;
            } else {
                tail.next = new ListNode(s);
                tail = tail.next;
                s = 0;
            }
        }
        return dummy.next;
    }
}

C++

class Solution {
public:
    ListNode* mergeNodes(ListNode* head) {
        ListNode dummy, *tail = &dummy;
        while (head) {
            if (head->val == 0) head = head->next; // skip leading `0`
            if (!head) break;
            int sum = 0;
            while (head->val != 0) { // sum numbers before the next `0`
                sum += head->val;
                head = head->next;
            }
            tail->next = new ListNode(sum); // append `sum`
            tail = tail->next;
        }
        return dummy.next;
    }
};