Arrays & Hashing

Use hash maps, sets, and array manipulation to solve problems involving lookups, frequency counting, and element relationships.

When to use

• Need O(1) lookup
• Counting frequencies
• Finding pairs/duplicates
• String manipulation

When NOT to use

• Need sorted order
• Need range queries
• Memory is severely constrained

Common traps

• Forgetting edge cases (empty array, single element)
• Off-by-one errors in indices
• Not handling duplicates

Key Invariant

Hash maps trade space for time — O(1) lookup at the cost of O(n) memory

Problems (63)

#5Check if All A's Appears Before All B's

#6Contains Duplicate

#8Palindrome Number

#9Longest Common Prefix

#13Single Number

#19Valid Anagram

#21Roman to Integer

#23Length of Last Word

#25Majority Element

#57Power of Two

#59Number of 1 Bits

#61First Unique Character in a String

#164Missing Number

#165Reverse Bits

#168Power of Four

#201Counting Sort

#232Happy Number

#233Excel Sheet Column Number

#236GCD and LCM

#260Contains Duplicate II

#261Isomorphic Strings

#262Word Pattern

#7Reverse Integer

#28Group Anagrams

#29Product of Array Except Self

#32Rotate Array

#33Set Matrix Zeroes

#34Spiral Matrix

#132Longest Consecutive Sequence

#162Single Number II

#163Single Number III

#166Sum of Two Integers

#167UTF-8 Validation

#190Rotate Image

#191Game of Life

#192Valid Sudoku

#193Diagonal Traverse

#220Zigzag Conversion

#221String to Integer (atoi)

#222Count and Say

#223Multiply Strings

#224Repeated DNA Sequences

#226Subarray Sum Equals K

#228Next Permutation

#231Wiggle Sort II

#234Factorial Trailing Zeroes

#235Integer to Roman

#237Sieve of Eratosthenes

#264Encode and Decode TinyURL

#275Find Kth Largest Element using Quickselect

#297Bitwise AND of Numbers Range

#299Total Hamming Distance

#302Rabin-Karp String Matching

#306Contiguous Array

#303Manacher's Algorithm