#https://leetcode.com/problems/merge-nodes-in-between-zeros
class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
class Solution:
    def mergeNodes(self, head: Optional[ListNode]) -> Optional[ListNode]:
        left = ListNode(-1, None)
        left.next = head
        curr = head
        val = 0
        while curr is not None:
            if curr.val == 0 and left.next != curr:
                left.next.val = val
                left = left.next
                val = 0
            val += curr.val
            curr = curr.next
        left.next = None
        return head