3分でAtCoder Beginner Contest 372 A-F【ゆっくり解説】

evima lab
21 Sept 202403:47

Summary

TLDRThe video covers six programming problems, offering explanations and Python code tips for each. Problem A involves removing dots from a string. Problem B explores expressing integers as sums of powers of 3. Problem C deals with string manipulation and substring counting. Problem D focuses on counting visible buildings in a skyline, utilizing stack-based optimization. Problem E introduces Union-Find for graph management and top-k queries. Problem F involves counting paths in a directed graph with dynamic programming tricks. The video ends with an announcement for the final episode in the next installment.

Takeaways

  • 😀 Problem A involves removing dots from a string of lowercase alphabets and outputting the concatenated result.
  • 🤔 Python's print function adds a newline by default but can be replaced with any string, including an empty one.
  • 🧮 Problem B asks to express a positive integer M as a sum of powers of 3, resembling base 3 conversion.
  • 🔄 Problem C requires counting occurrences of 'ABC' in a string after repeated character replacements, with optimization necessary due to input size.
  • 🏗️ Problem D involves calculating how many buildings can be seen to the east for each building in a row, using a stack for efficiency.
  • 📊 Union-Find is used in Problem E to manage connections in an undirected graph and answer queries about the kth largest vertex connected to another.
  • 🏗️ Problem E involves keeping track of the top 10 largest vertices during union-find merges to handle queries efficiently.
  • 🕒 Problem F focuses on counting paths of a specified length in a directed graph with up to 200,000 vertices.
  • ⚡ Optimization techniques like dynamic programming are mentioned for Problem F to handle larger datasets.
  • 📢 An announcement teases that the next episode will be the final one in the series.

Q & A

  • What is the task in Problem A?

    -The task in Problem A is to take an input string containing lowercase alphabets and dots, remove all the dots, and output the concatenated remaining string.

  • How can you manipulate the newline character in Python's print function?

    -In Python, you can replace the default newline character added by the print function with any string, including an empty one, by using the 'end' parameter.

  • What is the goal of Problem B?

    -The goal of Problem B is to express a given positive integer M as a sum of powers of 3, where M is up to 100,000 and you can use up to 20 numbers.

  • Why is expressing a number in base 3 considered unusual in the context of Problem B?

    -Expressing a number in base 3 is considered unusual because it's not a common base for number representation, and it requires a different approach than the more familiar base 10 or binary systems.

  • What optimization can be made in Problem C to avoid recounting the entire string each time?

    -In Problem C, instead of recounting the entire string each time, you can recount only around the changed part to optimize the process.

  • What is the time complexity limitation for Problem C?

    -The time complexity limitation for Problem C is set by a 2-second time limit, which allows for approximately 400 million loops.

  • How does the solution for Problem D utilize the concept of a stack?

    -The solution for Problem D uses a stack to keep track of visible buildings as it scans from right to left, adding the current building and removing shorter ones when a taller building is encountered.

  • What is the key to Problem E's solution involving an undirected graph?

    -The key to Problem E's solution is using the Union-Find algorithm to manage and quickly answer queries about the connectivity and the kth largest vertex in the connected component.

  • What is the strategy for Problem F when dealing with a large number of vertices?

    -The strategy for Problem F involves compressing the graph by focusing only on 'important' stations and using dynamic programming to count paths of specified lengths.

  • What is the significance of the announcement mentioned at the end of the transcript?

    -The announcement at the end of the transcript reveals that the next episode will be the final one, indicating the conclusion of the series.

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Related Tags
programming contestcoding solutionsPython tipsalgorithmsproblem-solvingbase 3string manipulationdynamic programmingUnion-Findgraph theory