Remove duplicates from sorted list II

Problem Given the head of a sorted linked list, remove ALL nodes whose value appears more than once, keeping only values that were already distinct.

Input / Output

• Input: head of sorted list.
• Output: head of the filtered list.

Constraints

• Up to 300 nodes classically; O(n) time, O(1) space, single pass preferred.

Example

• 1→2→3→3→4→4→5 → 1→2→5 (contrast with variant I, which keeps one copy: 1→2→3→4→5).

Expected approach

• Dummy head + prev pointer: if the node after prev starts a duplicate run (equal consecutive values), skip the whole run and splice prev.next past it WITHOUT advancing prev (the next candidate may also start a run); otherwise advance prev. prev only advances after a confirmed-distinct node — that's the subtlety. O(n)/O(1).