class Solution {
    public ListNode mergeNodes(ListNode head) {
        ListNode slow = head.next;
        ListNode fast = head.next;
        int sum=0;
        while(fast!=null){
            sum+=fast.val;
            if(fast.val==0){
                slow.val=sum;
                slow.next=fast.next;
                slow=slow.next;
                sum=0;
            }
            fast=fast.next;
        }
        return head.next;
    }
}