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Quick facts

• Difficulty: MEDIUM
• Problem: Product of Array Except Self
• Main tags: Array, Prefix Sum

What the problem is really asking

For each index, the answer should be the product of every number except the one currently sitting at that index.

If the input is [1, 2, 3, 4], then:

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Quick facts

• Difficulty: MEDIUM
• Problem: Longest Repeating Character Replacement
• Main tags: Hash Table, String, Sliding Window

What the problem is really asking

The input is a string and a budget k. One operation lets us replace any single character with any other character. The goal is to find the longest substring that can be turned into a string of all one repeated character using at most k replacements.

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Selected problem: TinyURL

Scope: Design a highly available URL shortening service that creates compact aliases, resolves them with very low latency, and records click analytics without putting the redirect path at risk.

Also see https://wiki.derricklin.net/software-development/System%20Design%20Interview/#url-shortener

Problem framing

This is the classic URL shortener problem: map a short code to a long URL, make redirects fast, and survive a very read-heavy workload with occasional viral hot keys.

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Quick facts

• Difficulty: HARD
• Problem: Number of Visible People in a Queue
• Main tags: Array, Stack, Monotonic Stack

What the problem is really asking

Each person is looking to the right. For every position, the task is to count how many people are visible before the view is blocked.

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Quick facts

• Difficulty: HARD
• Problem: Russian Doll Envelopes
• Main tags: Array, Binary Search, Dynamic Programming, Sorting

What the problem is really asking

Each envelope has a width and a height. One envelope can go inside another only if both dimensions are strictly smaller.

So the real question is:

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Quick facts

• Difficulty: MEDIUM
• Problem: Two Sum II - Input Array Is Sorted
• Main tags: Array, Two Pointers, Binary Search

What the problem is really asking

The input array is already sorted in non-decreasing order, and the goal is to find the two numbers whose sum equals the target.

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Quick facts

• Difficulty: MEDIUM
• Problem: Number of Islands
• Main tags: Array, Depth-First Search, Breadth-First Search, Union-Find, Matrix

What the problem is really asking

The grid is a map of water and land:

• "1" means land
• "0" means water

An island is one connected piece of land using only the four cardinal directions: up, down, left, and right.

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Quick facts

• Difficulty: MEDIUM
• Problem: Kth Largest Element in an Array
• Main tags: Array, Divide and Conquer, Sorting, Heap (Priority Queue), Quickselect

What the problem is really asking

This problem asks for the value that would appear in position k if the array were sorted in descending order.

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Quick facts

• Difficulty: MEDIUM
• Problem: Meeting Rooms II
• Main tags: Array, Two Pointers, Greedy, Sorting, Heap (Priority Queue)

What the problem is really asking

Each interval represents one meeting with a start time and an end time.

The question is not really about building a calendar or labeling rooms. It is asking:

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Quick facts

• Difficulty: MEDIUM
• Problem: Top K Frequent Elements
• Main tags: Array, Hash Table, Bucket Sort, Heap

What the problem is really asking

The input is an array of integers and a number k.

The task is not to sort the array itself. It is to: