121.Random Tic-Tac-Toe
★★★combinatoricsJane Street
Two players fill a tic-tac-toe board by always choosing uniformly at random among the empty squares, stopping as soon as someone makes three in a row. What is the probability that the game ends in a draw?
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Recurse over board positions, weighting each available move equally, and stop as soon as a line is completed.
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