Remove duplicates from a singly linked list

You are given the head of a **singly linked list** of integers. Modify the list **in place** so that it contains **only the first occurrence** of each value (i.e., remove any node whose value has already appeared earlier in the list). - Each node has fields: - `data` (integer) - `next` (reference to next node, or `null` at the tail) - The **relative order** of the remaining nodes must stay the same. Return the head of the updated linked list. ### Examples 1. Input: `3 -> 4 -> 3 -> 6` Output: `3 -> 4 -> 6` 2. Input: `3 -> 4 -> 3 -> 2 -> 6 -> 1 -> 2 -> 6` Output: `3 -> 4 -> 2 -> 6 -> 1`