Array Data Structure Guide

In this article, we introduce array, implementation in different popular languages, its basic operations and commonly seen problems / interview questions.

• An array stores items (in case of C/C++ and Java Primitive Arrays) or their references (in case of Python, JS, Java Non-Primitive) at contiguous locations.
• It offers mainly the following advantages over other data structures.
  Random Access : i-th item can be accessed in O(1) Time as we have the base address and every item or reference is of same size.
  Cache Friendliness : Since items / references are stored at contiguous locations, we get the advantage of locality of reference.
• It is not useful in places where we have operations like insert in the middle, delete from middle and search in a unsorted data.
• It is a fundamental and linear data structure using which we build other data structures like Stack Queue, Deque, Graph, Hash Table, etc.

Learn Basics of Array:

• Introduction to Arrays
• Applications of Array

Array in Different Language:

• Arrays in C
• Vector in C++ STL
• Arrays in Java
• ArrayList in Java
• List in Python
• Arrays in C#
• Arrays in JavaScript

Basic Problems on Array:

• Print Alternates
• Linear Search
• Largest Element
• Second Largest
• Largest Three
• Leaders in an array
• Check if Sorted
• Remove Duplicates from Sorted
• Generate all Subarrays
• Reverse an Array
• Rotate an Array
• Zeroes to End
• Min Increments to Make Equal
• Min Cost to Make Size 1

Prerequisite for the Remaining Problems

1. Binary Search
2. Selection Sort, Insertion Sort, Binary Search, QuickSort, MergeSort, CycleSort, and HeapSort
3. Sort in C++ / Sort in Java / Sort in Python / Sort in JavaScript
4. Two Pointers Technique
5. Prefix Sum Technique
6. Basics of Hashing
7. Window Sliding Technique

Easy Problems on Array:

• Distinct Elements
• Duplicate within K Distance
• Make Even Positioned Greater
• Sum of all Subarrays
• Stock Buy and Sell – Multiple Transactions
• Single Among Doubles
• Missing Number
• Missing and Repeating
• Only Repeating from 1 to n-1
• Sorted Subsequence of Size 3
• Binary Sort
• Stable Binary Sort
• Stable Binary Sort with O(1) Space
• Sort in Wave Form
• Max Subarray Sum
• K Concatenation Max Subarray Sum
• Max Subarray Product
• Equilibrium index
• Two Sum - Find if there is a Pair
• Two Sum - Closest Pair [More problems on 2 Sum in Medium Section]
• Chocolate Distribution Problem
• Union of two arrays
• Intersection of two arrays
• Union of two sorted arrays
• Intersection of two sorted arrays

Medium Problems on Array:

• Make arr[i] = i
• Sort an array of 1 to n
• Reorder according to given indexes
• Minimum Swaps to Sort
• Min in a Sorted Rotated
• Max in a Sorted Rotated
• Rotation Count in Rotated Sorted
• Search in a Sorted Rotated
• Nearly Sorted (or K sorted)
• Product Except Self
• K-th Largest Sum Subarray
• Smallest missing number
• Smallest subarray with sum greater than x
• Inversion count
• Merge Overlapping Intervals
• Sort an array of 0s, 1s and 2s
• Merge with O(1) extra space
• Majority Element
• Peak element
• Count possible triangles
• Sub-array with given sum
• Longest Subarray with Equal 0s and 1s
• Longest Common Span in Two Binary Arrays
• Construct an array from its pair-sum array
• 2 Sum - All Pairs
• 2 Sum - Distinct Pairs
• 3 Sum - Find Any
• 3 Sum - Closest Triplet
• 4 Sum - Find Any [More problems on 4 Sum in Hard Section]

Hard Problems on Array:

• Surpasser Count
• Trapping Rain Water
• Top K Frequent Elements
• Kth Missing Positive Number in a Sorted Array
• Stock Buy and Sell - At Most K Transactions
• Stock Buy and Sell - At Most 2 Transactions
• Median in a Stream
• Smallest Difference Triplet from 3 arrays
• Max occurred in n ranges
• 3 Sum - Distinct Triplets
• 3 Sum - All Triplets
• 4 Sum - Distinct Quadruples
• 4 Sum - All Quadruples
• 4 Sum - Closest Quadruple

Expert Problems for Competitive Programmers

• MO’s Algorithm
• Square Root (Sqrt) Decomposition Algorithm
• Sparse Table
• Range sum query using Sparse Table
• Range Minimum Query (Square Root Decomposition and Sparse Table)
• Range LCM Queries
• Merge Sort Tree for Range Order Statistics
• Minimum number of jumps to reach end
• Space optimization using bit manipulations
• Max value of Sum( i*arr[i]) with only rotations allowed

Quick Links :

• ‘Practice Problems’ on Arrays
• Top Array Interview Questions
• ‘Quizzes’ on Arrays