The preorder traversal of a binary search tree is given by $12, 8, 6, 2, 7, 9, 10, 16, 15, 19, 17, 20$.
Then postorder traversal will be:
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$2, 6, 7, 8, 9, 10, 12, 15, 16, 17, 19, 20$
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$2, 7, 6, 10, 9, 8,15, 17, 20, 19, 16, 12$
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$7, 2, 6, 8, 9, 10, 20, 17, 19, 15, 16, 12$
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$7, 6, 2, 10, 9, 8, 15, 16, 17, 20, 19, 12$