easy

25 minIntern through senior

When to use. Random access, contiguous ranges, or any problem that becomes a scan once you pick the right view.

Complexity. Time Access O(1); scan O(n); insert/delete at front O(n). Space O(1) extra for in-place edits; O(n) if you copy.

Who gets asked. Interns implement scans and in-place edits. Mid/senior interviews assume this and jump to window or prefix patterns.

Prereq. Big O for loops and index access.

Arrays & Strings: The Building Blocks

Learn about the most fundamental data structures in programming and how to use them effectively.

What Are Arrays?

An array is the simplest and most widely used data structure. You can think of it as a list of items stored in a contiguous block of memory. Each item, or "element," has a numbered position, called an "index," starting from 0.

Imagine a street with a row of mailboxes. Each mailbox has a number (the index) and contains a piece of mail (the element). If you know the mailbox number, you can go directly to it without checking the others. This direct access is what makes arrays so fast for retrieving elements. Access is O(1); inserting at the front is O(n) because every later index shifts.

In 2026 interviews, arrays are the substrate for two pointers, sliding windows, and prefix sums — not a recap of language collection APIs. Interns implement scans and in-place edits; later rounds assume this and jump to those patterns.

Interactive Array

Add, remove, and access elements to see how an array works.

Apple

Banana

Cherry

Visualizing Time Complexity

See why some array operations are slower than others. "Start" and "End" refer to the beginning and end of the array.

Access

Accessing an element by its index is instantaneous because its memory location is calculated directly.

O(1)

Common Array Patterns

Two Pointers

Using two pointers to iterate through an array from different ends until they meet. Ideal for problems involving sorted arrays or palindromes. Full pattern lesson: two pointers.

Problem: pair with target sum

A sorted array. Is there a pair of distinct positions that add to a target? Here the target is 22. Two pointers, O(n) after the sort.

Analogy: Two friends start at opposite ends of a library aisle, walking towards each other to find two books whose page counts sum to a specific value.

L
↑

↓
R

Start with pointers at both ends of the sorted array.

Sliding Window

A window that grows and shrinks over a sequence. Used for longest/shortest contiguous ranges. Full pattern lesson: sliding window.

Problem: Maximum Sum Subarray

Given an array of integers, find the maximum sum of any contiguous subarray of size 'k'. Here, k is 3.

Analogy: Imagine a train with 3 windows. As it moves along the tracks (the array), you want to find the section of track where the sum of numbers seen through the windows is the highest.

Start with an empty window.

Window Sum:

| Max Sum Found:

Quick Quiz: Test Your Knowledge

What is the time complexity to get the first element of an array (`arr[0]`)?

What is the time complexity to add an element to the beginning of an array (`arr.unshift(newItem)`)?

You need to find if a value exists in an unsorted array. What is the worst-case time complexity?

Coding Challenge

Select a Problem

For each index i, the product of every value except nums[i]. Linear time, no division. Prefix from the left times suffix from the right.

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