Stack

• List of items in the order they were added• Only two operations– Push– Pop

• Push adds an item to the front of the stack• Pop removes an item from the front of the

stack• This makes a stack last-in-first-out (LIFO)

Stack

• Push

• Pop

12

Next

Head 43

Next

65

Next

23

Next

12

Next

Head 43

Next

65

Next

23

Next

33

Next

12

Next

Head 43

Next

65

Next

23

Next33

Next

Stack in Postfix (RPN) Math• The infix expression 5 + ((1 + 2) *4) −3 can be written in RPN as 5 1 2 + 4 * + 3 - • The expression is evaluated left-to-right, with the inputs interpreted as shown in

the following table• When a computation is finished, its result remains as the top (and only) value in

the stack; in this case, 14.

Input Operation Stack Comment5 Push operand 51 Push operand 5, 12 Push operand 5, 1, 2+ Add 5, 3 Pop two values (1, 2) and push result (3)4 Push operand 5, 3, 4* Multiply 5, 12 Pop two values (3, 4) and push result (12)

+ Add 17 Pop two values (5, 12) and push result (17)3 Push operand 17, 3− Subtract 14 Pop two values (17, 3) and push result (14)