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Quick facts

• Difficulty: MEDIUM
• Problem: Insert Delete GetRandom O(1)
• Main tags: Array, Hash Table, Math, Design, Randomized

What the problem is really asking

The task is to design a set-like data structure with three operations:

• insert(val) adds a value only if it is not already present.
• remove(val) deletes a value only if it is present.
• getRandom() returns one currently stored value, with every stored value equally likely.

The important constraint is that all three operations should run in average O(1) time.

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Quick facts

• Difficulty: HARD
• Problem: Basic Calculator
• Main tags: Math, String, Stack, Recursion

What the problem is really asking

The input is a string expression that contains non-negative integers, +, -, parentheses, and spaces. The task is to return the final numeric value without using Python’s built-in expression evaluator.

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Quick facts

• Difficulty: MEDIUM
• Problem: Min Stack
• Main tags: Stack, Design

What the problem is really asking

The API looks simple: push, pop, top, and getMin.

The real requirement is hidden in getMin: it must return the current minimum in constant time, not by scanning the whole stack.

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Quick facts

• Difficulty: MEDIUM
• Problem: Binary Tree Level Order Traversal
• Main tags: Tree, Breadth-First Search, Binary Tree

What the problem is really asking

The task is to read a binary tree one layer at a time.

Instead of visiting nodes in a depth-first order such as root, left subtree, right subtree, the problem wants the tree grouped by depth:

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Quick facts

• Difficulty: MEDIUM
• Problem: Separate Squares I
• Main tags: Array, Binary Search

What the problem is really asking

This problem looks geometric, but the core idea is much simpler than it first appears.

The task is to place a horizontal line y = cut so that:

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Quick facts

• Difficulty: MEDIUM
• Problem: Coin Change II
• Main tags: Array, Dynamic Programming

What the problem is really asking

The input gives a target amount and a list of coin denominations. Each denomination can be used as many times as needed.

The task is to count how many distinct combinations can make the amount exactly. It is not asking for the minimum number of coins, and it is not asking whether the amount is possible at all.

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Quick facts

• Difficulty: MEDIUM
• Problem: Coin Change
• Main tags: Array, Dynamic Programming, Breadth-First Search

What the problem is really asking

The input gives a set of coin denominations and a target amount. Each coin can be used as many times as needed.

The goal is not to count how many different ways the amount can be formed. The goal is to find the fewest coins needed to reach the exact amount. If the amount cannot be formed exactly, the answer is -1.

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Quick facts

• Difficulty: HARD
• Problem: Sliding Window Maximum
• Main tags: Array, Queue, Sliding Window, Heap (Priority Queue), Monotonic Queue

What the problem is really asking

The input gives an array and a window size k. Imagine placing a length-k window over the first k elements, recording the maximum value inside that window, then moving the window one step to the right and repeating until the end.

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Quick facts

• Difficulty: MEDIUM
• Problem: Max Consecutive Ones III
• Main tags: Array, Binary Search, Sliding Window, Prefix Sum

What the problem is really asking

This problem sounds like a bit-flipping question, but the useful reframe is simpler:

find the longest contiguous subarray that contains at most k zeroes.

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Quick facts

• Difficulty: MEDIUM
• Problem: Maximum Product Subarray
• Main tags: Array, Dynamic Programming

What the problem is really asking

This problem asks for the contiguous subarray whose product is as large as possible.

That sounds close to Maximum Subarray, but products behave very differently from sums. With sums, a very negative running total is usually bad news. With products, a very negative running product can suddenly become the best thing to keep if the next number is also negative.