Evaluate the following postfix expression. Assume the following a = 7, b = 4, C = 3d = 1, e = 5 Notice that -c means negative c. Your answer should be a number. ab C.de + Why do we have to define a Collection like a Stack? Since ArrayStack actually is implemented with an array, why don't we simply use an array instead of going through the trouble of defining a stack with an array