A binary expression tree is one in which the interior nodes are operators, such as + or -, and the leaves are numbers. For example:
```                     +
/     \
-       +
/   \    / \
-     4  8  10
/ \
16  5
```
This expression tree represents the arithmetic expression:
```     ((16-5) - 4) + (8+10)
```
which evaluates to 25. An expression tree can be evaluated by evaluating the left subtree, then the right subtree, and finally performing the indicated operation on the values returned by evaluating the left and right subtrees. A leaf evaluates to its number. For example, in the above expression tree, the + node with the leaves 8 and 10 evaluates to 18 since its left subtree evaluates to 8 and its right subtree evaluates to 10. Similarly the root + node evaluates to 25 because its left subtree evaluates to 7 and its right subtree to 18.

You are given the following class declaration for an expression tree node:

```class ExpNode {
protected:
double value;
char operation;
bool leaf;
ExpNode *left;
ExpNode *right;
public:
ExpNode(double v) : value(v), leaf(true) {}
ExpNode(char opn, ExpNode *leftChild, ExpNode *rightChild)
: operation(opn), left(leftChild), right(rightChild), leaf(false) {}
double evaluate();
};
```
Your task is to write the method named evaluate. It should evaluate the subtree rooted at that ExpNode and return the value of computed expression as a double. If the node is a leaf, then the leaf variable will be true and the value field will be set to a number. If the node is an operation node, then leaf will be false and the operation field will contain the node's operator, which may be one of '+', '-', '*', and '/'.

If the node is a leaf, evaluate will return the number associated with the leaf. Otherwise evaluatewill evaluate the left and right subtrees, perform the operation indicated by the operation field on the results returned by the left and right subtrees, and return this value.

We have provided a testdriver in main that reads in a prefix arithmetic expression, constructs an expression tree from it, and calls the root node's evaluate method. Some sample inputs would be:

```5                // evaluate should return 5
* 3 4.2          // evaluate should return 12.6
- 4 9            // evaluate should return -5
* + 3 6 4        // evaluate should return 36. The input represents the
//   expression (3 + 6) * 4
/ * 8 5 - 4 2    // evaluate should return 20. The input represents the
//   expression (8 * 5) / (4 - 2)
```