listsstacksand queues two numbers (symbolsthat are popped from the stackand the result is pushed onto the stack for instancethe postfix expression + is evaluated as followsthe first four symbols are placed on the stack the resulting stack is topofstack nexta '+is readso and are popped from the stackand their sum is pushed topofstack topofstack next is pushed now '*is seenso and are poppedand is pushed topofstack