You are given the `head` of a linked list, which contains a series of integers separated by `0`'s. The beginning and end of the linked list will have `Node.val == 0`.

For every two consecutive `0`'s, merge all the nodes lying in between them into a single node whose value is the sum of all the merged nodes. The modified list should not contain any `0`'s.

Return the `head` of the modified linked list.

Example 1:

    Input: head = [0,3,1,0,4,5,2,0]
    Output: [4,11]
    Explanation:
    The above figure represents the given linked list. The modified list contains
    - The sum of the nodes marked in green: 3 + 1 = 4.
    - The sum of the nodes marked in red: 4 + 5 + 2 = 11.

Example 2:

    Input: head = [0,1,0,3,0,2,2,0]
    Output: [1,3,4]
    Explanation:
    The above figure represents the given linked list. The modified list contains
    - The sum of the nodes marked in green: 1 = 1.
    - The sum of the nodes marked in red: 3 = 3.
    - The sum of the nodes marked in yellow: 2 + 2 = 4.

Constraints:

- The number of nodes in the list is in the range `[3, 2 * 105]`.
- `0 <= Node.val <= 1000`
- There are no two consecutive nodes with `Node.val == 0`.
- The beginning and end of the linked list have `Node.val == 0`.