Hash maps and sets

The concept

A hash map stores values by key, and a set stores just the keys, both with an average lookup of O(1). That single property turns an O(n²) double loop into one pass in dozens of interview questions: "find two numbers that add to a target", "the first non-repeating character", "is there a duplicate", "group the anagrams".

The moving parts: adding a key, checking whether a key exists, reading one back, and counting with counts[x] = counts.get(x, 0) + 1. For a duplicate check a set is enough; for counting or grouping you need the map. In JavaScript a Map is safer than an object when the keys are numbers or come from user input.

def two_sum(nums, target):
    seen = {}                          # value -> index
    for i, n in enumerate(nums):
        if target - n in seen:
            return [seen[target - n], i]
        seen[n] = i
    return []

# "Is there a duplicate?" needs only a set:
def has_duplicate(nums):
    return len(set(nums)) != len(nums)

# A set tells you a value EXISTS; a map tells you WHERE.