A binary search tree is a binary tree where in each node: * The left subtree contains only nodes with keys less than the node's key (the "data"/"value" of the node). * The right subtree contains only nodes with keys greater than the node's key. * Both subtrees are also binary search trees. It follows that each key is distinct. Binary search trees are especially useful in constructing other data structures such as sets, multisets and associative arrays.