The hash map pattern: how to spot it

Updated Oct 15, 2026

A hash map trades memory for time: remember what you have seen, keyed by the thing you will look up later, and a nested loop becomes one pass. Most interview problems use it for one of three jobs: finding a partner value, counting, or grouping.

When to reach for it

• You would otherwise compare every pair, and the pair is defined by a value you can compute (a complement, a difference, a key).
• The question is about how many times something appears, or whether two things have the same counts.
• Items that share some property need to end up together, and you can turn that property into a key.
• The input is unsorted and sorting it would lose the original indexes you need to return.

seen = {}
for i, x in enumerate(items):
    want = partner_of(x)        # e.g. target - x
    if want in seen:
        return seen[want], i
    seen[x] = i                 # store after checking, so x can't pair with itself

3 LeetCode hash maps problems, easiest first

1. Two Sum easy

   One pass: store what you have seen, query the complement.

2. Valid Anagram easy

   Same letters iff the two frequency maps are equal.

3. Group Anagrams medium

   The anagram key is the grouping key. Do not overwrite the list under it.

Skills these problems use

Each one has a free lesson with worked examples and a drill.

• Two-sum with a value-to-index map
• Check whether a map contains a key
• Get the value for a key
• Build a frequency map
• Compare two frequency maps
• Group values by a key
• Append to a list under a key

All hash map lessons

Other patterns

• Hash set
• Array state
• Two pointers
• Sliding window