Julian and Johan play a game with 2n cards, always a winner
Source: 2008 Dutch IMO TST P2
January 10, 2020
combinatoricsgame strategywinning strategygame
Problem Statement
Julian and Johan are playing a game with an even number of cards, say cards, (). Every card is marked with a positive integer. The cards are shuffled and are arranged in a row, in such a way that the numbers are visible. The two players take turns picking cards. During a turn, a player can pick either the rightmost or the leftmost card. Johan is the first player to pick a card (meaning Julian will have to take the last card). Now, a player’s score is the sum of the numbers on the cards that player acquired during the game.
Prove that Johan can always get a score that is at least as high as Julian’s.