PROBLEMS/ARRAYS & HASHING

Arrays & Hashing LeetCode Problems

1013 hand-tagged Arrays & Hashing problems across 7 interview patterns (Frequency Counting / Hash Map Lookup, Hash-Backed Design, Prefix Sums, Kadane's Algorithm (Max Subarray), Sorting + Comparison, Complement Search (Two Sum Pattern), Matrix Traversal (Diagonal, Valid Sudoku, etc)), ranked by importance (how often each pattern shows up in real coding interviews). Work top-down within each pattern.

Frequency Counting / Hash Map Lookup

572 problems →

#	PROBLEM	DIFF	ELO	IMP	PATTERNS
1	Longest Consecutive Sequence	Medium	1412	91	Union-Find
2	Group Anagrams	Easy	1219	90	Sorting + Comparison
3	Contains Duplicate	Easy	978	82
4	Valid Anagram	Easy	1025	82	Sorting + Comparison
5	Majority Element	Easy	1052	82

+ 567 more Frequency Counting / Hash Map Lookup problems →

Hash-Backed Design

29 problems →

#	PROBLEM	DIFF	ELO	IMP	PATTERNS
1	LRU Cache	Medium	1580	93	Merge Linked Lists
2	Insert Delete GetRandom O(1)	Medium	1502	78	General Math
3	LFU Cache	Hard	2028	78	Reversal (Full and Partial)
4	Detect Squares	Hard	1841	72	General Math
5	Design HashMap	Easy	1038	69

+ 24 more Hash-Backed Design problems →

Prefix Sums

189 problems →

#	PROBLEM	DIFF	ELO	IMP	PATTERNS
1	Product of Array Except Self	Easy	1305	91
2	Subarray Sum Equals K	Easy	1324	91	Frequency Counting / Hash Map Lookup
3	Range Sum Query - Immutable	Easy	1028	90
4	Range Sum Query 2D - Immutable	Medium	1407	87	Matrix Traversal (Diagonal, Valid Sudoku, etc)
5	Range AdditionPREMIUM	Easy	1371	82

+ 184 more Prefix Sums problems →

Kadane's Algorithm (Max Subarray)

10 problems →

#	PROBLEM	DIFF	ELO	IMP	PATTERNS
1	Maximum Subarray	Easy	1200	93	Linear Recurrence DP
2	Best Time to Buy and Sell Stock	Easy	1053	92	Linear Recurrence DP
3	Maximum Sum Circular Subarray	Medium	1777	79	Prefix Sums
4	Find the Substring With Maximum Cost	Medium	1422	68	Frequency Counting / Hash Map Lookup
5	Maximum Score Of Spliced Array	Medium	1791	62	Linear Recurrence DP · Prefix Sums

+ 5 more Kadane's Algorithm (Max Subarray) problems →

Sorting + Comparison

129 problems →

#	PROBLEM	DIFF	ELO	IMP	PATTERNS
1	Sort an Array	Medium	1480	76
2	Next Permutation	Medium	1533	68	Opposite-Direction (Converging)
3	Divide Players Into Teams of Equal Skill	Easy	1323	63	Opposite-Direction (Converging) · Complement Search (Two Sum Pattern)
4	Make Lexicographically Smallest Array by Swapping Elements	Hard	2047	63	Union-Find · Sorting-Based Greedy
5	Number of Pairs Satisfying Inequality	Hard	2030	62	Binary Search on Answer Space · Prefix Sums

+ 124 more Sorting + Comparison problems →

Complement Search (Two Sum Pattern)

13 problems →

#	PROBLEM	DIFF	ELO	IMP	PATTERNS
1	Two Sum	Easy	1142	95	Frequency Counting / Hash Map Lookup
2	Max Number of K-Sum Pairs	Easy	1346	78	Opposite-Direction (Converging) · Frequency Counting / Hash Map Lookup
3	4Sum II	Easy	1372	72	Frequency Counting / Hash Map Lookup
4	Count Pairs That Form a Complete Day II	Easy	1385	68	Frequency Counting / Hash Map Lookup · Modular Arithmetic
5	Pairs of Songs With Total Durations Divisible by 60	Easy	1377	62	Modular Arithmetic

+ 8 more Complement Search (Two Sum Pattern) problems →

Matrix Traversal (Diagonal, Valid Sudoku, etc)

71 problems →

#	PROBLEM	DIFF	ELO	IMP	PATTERNS
1	Valid Sudoku	Easy	1267	76	Frequency Counting / Hash Map Lookup
2	Spiral Matrix	Medium	1418	76
3	Set Matrix Zeroes	Easy	1378	74	Frequency Counting / Hash Map Lookup
4	Island Perimeter	Easy	1076	68
5	Game of Life	Easy	1388	52	General Bit Manipulation

+ 66 more Matrix Traversal (Diagonal, Valid Sudoku, etc) problems →

EVERY PATTERN

Arrays & Hashing Two Pointers Stack Binary Search Sliding Window Linked List Trees Tries Backtracking Heap / Priority Queue Graphs Core DP Intervals Greedy Advanced Graphs Advanced DP Bit Manipulation Math & Geometry

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