# CodeStepByStep

## combineDuplicates

Write a function named `combineDuplicates` that manipulates a list of `ListNode` structures. The function accepts as a parameter a reference to a pointer to the front of a list, and modifies the list by merging any consecutive neighboring nodes that contain the same element value into a single node whose value is the sum of the merged neighbors. For example, suppose a pointer named `list` points to the front of a list containing the following values. The diagram shows the result of a call of `combineDuplicates(list);` on the list. The underlined areas represent the neighboring duplicate elements that are merged in the final result.

```{3, 3, 2, 4, 4, 4, -1, -1, 4, 12, 12, 12, 12, 48, -5, -5}       list

{3, 3, 2, 4, 4, 4, -1, -1, 4, 12, 12, 12, 12, 48, -5, -5}       combineDuplicates(list);

{6, 2, 12, -2, 4, 48, 48, -10}                                  result
```

If the list is empty or contains no duplicates, it should be unchanged by a call to your function.

Constraints: It is okay to modify the `data` field of existing nodes, if you like. Do not construct any new `ListNode` objects in solving this problem (though you may create as many `ListNode*` pointer variables as you like). Do not use any auxiliary data structures to solve this problem (no array, vector, stack, queue, string, etc). Your code must run in no worse than O(N) time, where N is the length of the list. Your code must solve the problem by making only a single traversal over the list, not multiple passes.

Assume that you are using the `ListNode` structure as defined below:

```struct ListNode {
int data;         // value stored in each node
ListNode* next;   // pointer to next node in list (nullptr if none)
}
```
Function: Write a C++ function as described, not a complete program.