def twoSum(nums, target): seen = {} # val -> index for i, num in enumerate(nums): complement = target - num if complement in seen: return [seen[complement], i] seen[num] = i return []

Mastering Hash Maps

The Hash Map pattern is essential for transforming O(N²) brute force solutions into efficient O(N) algorithms by trading space for time. It enables constant-time lookups and efficient counting/grouping operations.

Two Sum Pattern

Learn to find complements in O(1) time by checking the map before adding.

Frequency Counter

Master counting occurrences efficiently for anagrams and uniqueness checks.

Grouping

Learn to categorize items using canonical keys like sorted strings.

Common Interview Problems

01
Two Sum
The classic introduction. Find two numbers that add up to a target using a complement map.
02
Valid Anagram
Use a frequency counter to check if two strings contain the same characters.
03
Group Anagrams
Categorize strings by their sorted version as the hash map key.

Learn More

Explore our comprehensive guides to master every variation:

Complete Hash Map Guide View All Hash Map Tutorials

Hash Map Visualizer

Two Sum