Merge Nodes In Between Zeros — Pointer Walk

# Python
def mergeNodes(head):
    cur = head.next  # skip leading 0
    modify = head  # where to write sums
    total = 0
    while cur:
        if cur.val == 0:
            modify.next = cur if cur.next else None
            modify.val = total
            modify = modify.next
            total = 0
        else:
            total += cur.val
        cur = cur.next
    return head

// Java
public ListNode mergeNodes(ListNode head) {
    ListNode cur = head.next, modify = head;
    int total = 0;
    while (cur != null) {
        if (cur.val == 0) {
            modify.val = total;
            modify.next = cur.next != null ? cur : null;
            modify = modify.next;
            total = 0;
        } else total += cur.val;
        cur = cur.next;
    }
    return head;
}