usually stack is used.

example:

algorithm for postfix expression:

1) take eg. 123*+ . scan the expression, whenever operand comes push to stack.

2) when operator comes pop 2 operands from stack and compute result and push to stack again

3)stack-> push 1,2,3 => stack will be 3,2,1 (3 at the bottom)

4)pop 1,2. now do 1*2=2. push 2 to stack. now stack is 3,2

5) pop 2,3. now do 2+3=5.

6) now scanning the expression 123*+ is completed thus 5 is the result