Another kind of binary tree is an expression tree --- a
representation of arithmetic expressions. For example, the expression
2 + 3 can be represented as a binary tree with a root labeled with +.
The root would have two children labeled with 2 and 3. For more
complicated arithmetic expressions, the root of the tree is the
operation that is performed last. The left subtree and right subtree
of the root are the left and right operands of that last operation.
For example, in the expression (2 + 3) * (4 + 5), the * is performed
last, so the left and right subtrees represent (2 + 3) and (4 + 5),
respectively.
Note that in an expression tree, we do not need to store parentheses,
since the order of evaluation is implicit in the structure of the
tree.