How would you determine if a list contains duplicate values?

A simple O(n) solution uses a hash set. Iterate over the list, adding each element to a set. If an element is already present in the set, you have found a duplicate and can return True. If you finish the loop without finding a duplicate, return False. In Python:

def has_duplicates(lst):
    seen = set()
    for x in lst:
        if x in seen:
            return True
        seen.add(x)
    return False

The set provides O(1) average‑time lookups, so the overall time complexity is O(n) and the space complexity is O(n).