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Difficulty: MEDIUM
Problem: Best Time to Buy and Sell Stock II

Problem gist

The input is a list of stock prices where prices[i] is the price on day i. The goal is to make as much profit as possible by buying and selling any number of times.

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Quick facts

• Difficulty: MEDIUM
• Problem: Word Break
• Main tags: String, Hash Table, Dynamic Programming, Trie

What the problem is really asking

The input is a string s and a dictionary of valid words. The task is to decide whether s can be split into one or more dictionary words. Words may be reused, so the question is not about consuming the dictionary; it is about whether every character in the string can be covered by valid dictionary chunks.

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Difficulty: MEDIUM
Problem: Champagne Tower

Problem gist

A tower of glasses is stacked in rows. Row 0 has one glass, row 1 has two glasses, and so on. Each glass can hold exactly one cup. If a glass receives more than one cup, the extra amount spills equally into the two glasses directly below it.

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Difficulty: MEDIUM

Problem: Daily Temperatures

Problem gist

Given a list of daily temperatures, return a new list where each position says how many days must pass before a strictly warmer temperature appears. If no warmer future day exists, that position should be 0.

For example, if the temperatures are:

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Difficulty: MEDIUM
Problem: Network Delay Time

Problem gist

The input describes a directed, weighted graph. Each edge u -> v has a travel time w, and a signal starts at node k. The task is to return how long it takes until every node receives the signal. If some node can never receive it, return -1.

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Difficulty: MEDIUM

Problem: Group Shifted Strings

Problem gist

Each string can be shifted forward through the alphabet, wrapping from z to a. For example, shifting "abc" once gives "bcd", and shifting "az" once gives "ba". The task is to group strings that can become each other by applying the same shift amount to every character.

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Quick facts

• Difficulty: MEDIUM
• Problem: Random Pick with Weight
• Main tags: Array, Prefix Sum, Binary Search, Randomized

What the problem is really asking

The class receives an array of positive weights, where w[i] describes how likely index i should be picked. Calling pickIndex() should return one index at random, but not uniformly. Index i should be returned with probability:

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Difficulty: MEDIUM
Problem: Reverse Words in a String

Problem gist

The input is a string that contains words separated by spaces. The output should contain the same words in reverse order, with exactly one space between adjacent words and no leading or trailing spaces.

The important detail is that the function is not reversing letters inside each word. It is reversing the sequence of words. For example, the words ["the", "sky", "is", "blue"] become ["blue", "is", "sky", "the"].

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Difficulty: MEDIUM
Problem: Lowest Common Ancestor of a Binary Tree

Problem gist

Given the root of a binary tree and two existing nodes p and q, return their lowest common ancestor.

An ancestor can be the node itself. That detail matters: if p is above q, then p is the answer. “Lowest” means deepest in the tree, not smallest by value. This is a normal binary tree, so there is no useful ordering rule like there would be in a binary search tree.

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Quick facts

• Difficulty: HARD
• Problem: Word Ladder
• Main tags: Breadth-First Search, Hash Table, String, Graph

What the problem is really asking

The input gives a beginWord, an endWord, and a dictionary called wordList.

Each move can change exactly one letter, and every intermediate word must appear in wordList. The task is to return the number of words in the shortest valid transformation sequence from beginWord to endWord. If no such sequence exists, return 0.