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Team Selection Test

Turkey counting and probability

Problem

Alice and Bob play a game on a board using cards numbered from through . At each step, Alice chooses a card and Bob places it on an empty square of the board. Bob wins the game when numbers on the cards on the board are in an increasing order after steps where , otherwise Alice wins. Find all pairs for which Bob can guarantee to win the game.
Solution
Bob wins for all pairs of if and for all pairs if .

Let us show that Bob wins in all cases listed above. If then and the result is trivial. Suppose that Bob wins for when . Bob places the first card on a square numbered . The square divides the whole board into two parts. Let be the number on the first card chosen by Alice. After the first move all cards numbered less than will be placed to the left part and all cards numbered greater than will be placed to the right part. Note that both parts having sizes not less than and by assumption Bob has a winning strategy for remaining moves and we are done.

If and , then Bob just places the card numbered to the square numbered .

Now we show that Alice wins in all remaining cases. Alice's strategy: Let us line the cards in increasing order. Suppose Alice's -th move is a card numbered . Bob places the card numbered into some square. After Bob's -th move, the set of all vacant squares of the board is naturally decomposed into connected components. The card divides the connected component into two parts, say and . Alice chooses the part not exceeding the other one in length. The set of remaining cards is also naturally decomposed into connected components. If the part is then Alice will choose a card from connected component of remaining cards ending at for the next step, if the part is then Alice will choose a card from the connected component of remaining cards starting at for the next step. In her -th move, if she needs to choose a card from component she chooses a card numbered . It can be readily seen that Alice wins in all remaining cases if she starts with a card numbered .
Final answer
Bob can guarantee a win for all pairs with k between 1 and 10 and m at least 2^k − 1, and for all pairs with k between 11 and 2012 and m at least 2012.

Techniques

Games / greedy algorithmsInduction / smoothing