typedef struct NODE *pNODE;
struct NODE {
	char op;
	int value;
	pNODE leftmostChild, rightSibling;
};

int eval(pNODE n)
{
	int val1, val2; /* values of first and second subtrees */

	if (n->op == 'i') /* n points to a leaf */
		return n->value;
	else { /* n points to an interior node */
		val1 = eval(n->leftmostChild);
		val2 = eval(n->leftmostChild->rightSibling);
		switch (n->op) {
			case '+': return val1 + val2;
			case '-': return val1 - val2;
			case '*': return val1 * val2;
			case '/': return val1 / val2;
		}
	}
}