1. This project deals with a simple kind of expression tree, in which there are two kinds of nodes:
(a) Leaf nodes, which contain a real number as their element;
(b) Non-leaf nodes, which contain either the character '+' or the character '*' as their element and have exactly two children.
For this project, implement a class for expression trees, including operations for building expression trees. Also include a recursive function to “evaluate” a non-empty expression tree using these rules:
(a) If the tree has only one node (which must be a leaf), then the evaluation of the tree returns the real number that is the node’s element;
(b) If the tree has more than one node and the root contains '+', then first evaluate the left subtree, then evaluate the right subtree and add the results. If the root contains '*', then evaluate the two subtrees and multiply the results.
For example, consider the small expression tree shown to the right. The left subtree evaluates to 3+7, which is 10. The right subtree evaluates to 14. So the entire tree evaluates to 10*14, which is 140.
