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Algorithm isBST(tree):
   Input: a binary tree whose nodes contains keys
   Output: (Is tree a binary search tree?, minimal key stored in tree, maximal key stored in tree)
if tree has one node then
     result (true, key of root of tree, key of root of tree)
else
     (isBST_left, min_left, max_left) isBST(left subtree of tree)
     (isBST_right, min_right, max_right) isBST(right subtree of tree)
     isBST isBST_left isBST_right max_left <= key of root of tree <= min_right
     min min(key of root of tree, min_left, min_right)
     max min(key of root of tree, max_left, max_right)
     result (isBST, min, max)
return result

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