Solutions for NOIP 2009: Password Decryption, Inverse GCD-LCM, Trade Profit, and Target Sudoku
Problem 1: Decrypting S Country's Military Cipher
Description
R and S are at war. R's spy, Xiao C, has obtained S's encryption rule: each uppercase letter maps to a unique uppercase letter (bijection). Given a known encrypted string and its original string, build the substitution table. Then translate a new intercepted encrypted string. If the ...
Posted on Mon, 05 Oct 2026 16:17:26 +0000 by justinjkiss
Solving Sudoku and Minesweeper Combination Problems
Given a completed 9×9 Sudoku grid, the task requires preserving some digits (minimum one) while replacing others with mines. The condition is that each remaining digit must equal the count of adjacent mines in its 8 surrounding cells.
Solution Approach
Identify any non-edge cell containing '8' and convert all other cells to mines. This satisfie ...
Posted on Thu, 30 Jul 2026 16:45:49 +0000 by Anant