Given an array nums, return true if any value appears at least twice, and false if every element is distinct. Checking every pair works but revisits the same information repeatedly. A hash setHash SetA collection that supports checking whether a value is present, and adding a new value, both in average O(1) time — no scanning required. remembers every value seen so far, so each new number only needs a single O(1) lookup to know whether it's already appeared — turning the O(n²) comparison into a single O(n) pass.