How can you tell when a postfix notation is invalid just by looking at it?

1) Consider an algorithm that evaluates an arithmetic expression using "Postfix" notation using the "MyStack" class. You can use JAVA or pseudo-code to implement the following: It may be the case that the expression you supply is invalid, so an attempt to calculate the answer would result in an error. The evaluation program should include a routine to check whether the expression is valid and indicate to the user, e.g., through an error code, if the expression is invalid