double getValue( ExpNode *node ) { // Return the value of the expression represented by // the tree to which node     refers. Node must be non-NULL.
    if ( node->kind == NUMBER ) { // The value of a NUMBER node is the number it holds.
        return node->number;
    }
    else { // The kind must be OPERATOR. // Get the values of the operands and combine them
          // using the operator.
       double leftVal = getValue( node->left )
       double rightVal = getValue( node->right );

       switch ( node->op ) {
       case '+': return leftVal + rightVal;
       case '-': return leftVal - rightVal;
       case '*': return leftVal * rightVal;
       case '/': return leftVal / rightVal;
       }
    }
} // end getValue()

double getValue( ExpNode *node ) {
    if (node==NULL)
        return (double)INT_MAX; //include limits.h and don't use the value INT_MAX in any node

    //rest code
}