Let’s analyze the values of the nodes in this game tree.
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Nodes (7) and (9) represent board positions where Max (player X) wins with three X’s in a row. These positions have a value of +1.
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Nodes (5), (6), (8), and (10) correspond to positions where Min (player O) only needs to place an O in the remaining cell to win. These positions are practically over, and we assign them a value of -1.
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Now let’s consider nodes (2)–(4), which are one level higher towards the root. Both children of node (2) (i.e., nodes (5) and (6)) lead to Min’s victory, so we confidently assign a value of -1 to node (2).
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For node (3), the left child (7) leads to Max’s victory (+1), while the right child (8) leads to Min winning (-1). Since it’s Max’s turn to play, she will choose the left child (node 7). Therefore, every time we reach the board position in node (3), Max can ensure victory, and we assign the value +1 to node (3).
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Similarly, for node (4), Max can always choose where to place her X, ensuring victory. Thus, we attach the value +1 to node (4).