Count the number of connected components in a linked list defined by a given set of values using a single traversal and O(1) HashSet lookups. A clean O(n) interview problem at Google, Amazon, and Bloomberg that tests your ability to convert a graph concept into a simple linear scan.
LeetCode 217 asks whether any value repeats in an array. The hash set answer runs in O(n) time and O(n) space. This guide compares it against sorting and brute force, shows why hash sets degrade to O(n) in the worst case, and works through the Contains Duplicate II and III follow-ups interviewers actually ask next.