[Java] Remove consecutive nodes with zero sum from linked list

Given a singly linked list, remove all consecutive nodes that sum up to 0 from it.

Examples:

0 return null

6, -6 return null

10, 4, -4 return 10

10, 4, -3, -1, 2 return 10, 2

We use two pointers, one is sitting before the head, the other walks the list.
We keep summing values of nodes up and each time we store in a map the pair(sum up to this node, node) then we check the current sum, we can have the following cases:
(1) sum is 0: this means from start to end all elements sum to 0, we need to remove all of them and we need to carefully handle the head:
- if start pointer is null, it means this 0 sum includes the head, we will update it
- if start pointer is not null, it means this 0 sum occurs from a node (which might be the head), we update the NEXT of it
(2) sum is a value we already encountered: this means that AFTER the last node where we saw that sum until the end all elements sum to 0, we update the NEXT of the current start to cut them out
(3) otherwise we store the value in the map and move on to the next element

Whenever we perform a cut, we rest the counter to 0 and clear the map since we remove all consecutive nodes whose sum is 0, therefore after a cut we restart from the last node we kept.

This works in O(N) time and uses O(N) space.

You can check my implementation of removeNodesWithZeroSum on my Gist along with some tests in RemoveNodesWithZeroSumJTests.