Arrays

A step-by-step guide to mastering arrays for DSA beginners, with 50+ curated LeetCode problems, ordered from easiest to medium difficulty, including prerequisites and concepts to know before solving each problem.

Step 0: Fundamentals (Basics of Arrays)

Goal: Understand arrays, indexing, traversal, and basic operations.

# Problem Difficulty Prerequisites / Concepts
1 Find Maximum / Minimum in Array Easy Array traversal, comparing elements, iteration
2 Sum of Array / Running Sum of 1D Array Easy Looping, cumulative sum, array indexing
3 Linear Search / Search Insert Position Easy Traversal, condition check, returning index
4 Contains Duplicate Easy Brute-force, optional HashMap for optimization
5 Maximum Product Subarray Medium Tracking max/min while traversing, basic DP concept

Step 1: Basic Array Manipulations

Goal: Learn in-place changes, swapping, and two-pointer basics.

# Problem Difficulty Prerequisites / Concepts
6 Remove Duplicates from Sorted Array Easy Two pointers, in-place modification
7 Rotate Array Easy Array reversal, swapping, indexing
8 Move Zeroes Easy Two pointers, shifting elements in-place
9 Reverse String / Array Easy Two pointers, swapping elements
10 Replace Elements with Greatest Element on Right Easy Array traversal, keeping track of max value
11 Merge Sorted Array Easy Two pointers, merging arrays in-place

Step 2: Search & Sum Patterns

Goal: Learn brute-force and hash-based solutions.

# Problem Difficulty Prerequisites / Concepts
12 Two Sum Easy Brute-force, HashMap for O(n) optimization
13 Single Number Easy XOR trick or HashMap, counting frequency
14 Intersection of Two Arrays II Easy HashMap for counting, array traversal
15 Running Sum of 1D Array Easy Prefix sum, array traversal
16 Find All Numbers Disappeared in an Array Easy Index mapping, in-place marking
17 Majority Element Easy Counting frequency, HashMap or Boyer-Moore voting algorithm
18 Find Pivot Index Easy Prefix sums, array traversal

Step 3: Two-Pointer Technique

Goal: Solve problems efficiently by scanning from both ends or merging.

# Problem Difficulty Prerequisites / Concepts
19 Container With Most Water Medium Two pointers, maximizing area, left/right moves
20 Valid Palindrome Easy Two pointers from ends, skipping invalid characters
21 Squares of a Sorted Array Easy Two pointers, merging sorted arrays
22 Two Sum II - Input Array Sorted Easy Two pointers on sorted array
23 3Sum Medium Sorting + two pointers, avoiding duplicates
24 3Sum Closest Medium Sorting + two pointers, minimize difference
25 Remove Element Easy Two pointers, in-place deletion

Step 4: Sliding Window / Subarray Problems

Goal: Learn efficient subarray handling techniques.

# Problem Difficulty Prerequisites / Concepts
26 Maximum Average Subarray I Easy Fixed-size sliding window, sum tracking
27 Min Size Subarray Sum Medium Variable-size sliding window, sum tracking
28 Longest Substring Without Repeating Characters Medium Sliding window + HashMap for counts
29 Subarray Sum Equals K Medium Prefix sum + HashMap, sum counting
30 Longest Subarray of 1’s After Deleting One Element Medium Sliding window, handling deletion
31 Maximum Consecutive Ones III Medium Sliding window with k flips
32 Permutation in String Medium Sliding window, frequency map, string to array mapping

Step 5: Sorting + Two-Pointer Patterns

Goal: Combine sorting with pointers for intermediate problems.

# Problem Difficulty Prerequisites / Concepts
33 Merge Intervals Medium Sorting by start time, merging ranges
34 Find First and Last Position of Element Medium Binary search, array boundaries
35 Search in Rotated Sorted Array Medium Modified binary search, sorted array logic
36 3Sum Smaller Medium Sorting + two pointers, counting triplets
37 Sort Colors Medium Dutch National Flag, two pointers
38 Intersection of Two Arrays Easy Sorting + two pointers, set intersection

Step 6: Advanced Beginner Patterns

Goal: Combine all techniques learned so far.

# Problem Difficulty Prerequisites / Concepts
39 Maximum Product Subarray Medium Tracking max/min, handling negative numbers
40 Trapping Rain Water Medium Two-pointer or left/right max arrays, water trapping logic
41 Longest Consecutive Sequence Medium HashSet for O(n) sequence detection
42 Find Median from Data Stream Hard Heap / priority queue, dynamic array
43 Candy / Jump Game / Gas Station Medium Greedy, DP, array traversal patterns
44 Product of Array Except Self Medium Prefix / suffix product, no division
45 Top K Frequent Elements Medium HashMap + Heap, frequency counting
46 Sort an Array Medium Implementing QuickSort / MergeSort
47 Find All Duplicates in an Array Medium In-place marking, negative indexing trick
48 Set Matrix Zeroes Medium Matrix traversal, in-place marking
49 Rotate Image Medium Matrix manipulation, transpose + reverse
50 Spiral Matrix Medium Boundary simulation, traversal layers
51 Diagonal Traverse Medium Index calculation, array mapping

This covers 50+ array problems and teaches all beginner-friendly array patterns, including: