AtCoder Beginner Contest 462: A peek at problem-solving approaches
AtCoder’s Beginner Contest 462 wrapped up with five problems that tested everything from string parsing to combinatorics. One participant recently shared their post-contest review, walking through the thought process and Python code for each problem from A to E, offering practical examples for others to learn from.
From strings to sets: extracting digits and mapping gifts
Problem A asked to pull all digits from a mixed string and output them in order. The author first checked each character against the list of digits “0” through “9” before discovering Python’s isdigit() method, which streamlined the check. Problem B turned to social networks of sorts: given who sent gifts to whom among N people, the task was to list every sender for each recipient. The solution built a dictionary mapping each person to a list of senders, iterating twice—once to initialize and once to populate—before printing the counts and lists.
Geometry, crime, and efficient counting
Problem C moved into the plane, asking how many points lie outside any axis-aligned rectangle formed by the smallest x and smallest y among earlier points. The author sorted points by x-coordinate and walked through them while tracking the minimum y seen so far; whenever a new minimum y appeared, the corresponding point could not be covered by any prior rectangle, incrementing the count. Problem D introduced a crime scenario: finding valid pairs of suspects who were both inside a facility for the entire duration of a D-unit crime window. The solution used a sweep-line array to mark when suspects entered and exited the “valid” window (T_i − D + 1 to S_i), computed prefix sums to count how many were present at each time, then applied a simple combination formula to sum pairs across all times.
Takeaways for competitive programmers
The participant emphasized clarity over cleverness, noting how small Python utilities like isdigit() and built-in sorting simplified otherwise manual loops. For larger constraints, prefix sums and incremental updates kept solutions efficient. The write-up serves as both a refresher for past contestants and a concise reference for anyone tackling similar challenges.
Source: DEV Community. AI-assisted editorial synthesis — TechnoExpress.