Evaluate a Postfix expression
=============================

2351-*4/+1-


term  action                               stack      leftOp operator rightOp
 2    if, push 2 onto stack                2
 3    if, push 3 onto stack                2 3
 5    if, push 5 onto stack                2 3 5
 1    if, push 1 onto stack                2 3 5 1
 -    else pop, pop,                       2 3           5       -       1 
           perform operation, push result  2 3 4          
 *    else pop, pop,                       2             3       *       4
           perform operation, push result  2 12           
 4    if, push 4 onto stack                2 12 4
 /    else pop, pop,                       2            12       /       4
           perform operation, push result  2 3           
 +    else pop, pop,                       2 3           2       +       3
           perform operation, push result  5            
 1    if, push 1 onto stack                5 1
 -    else pop, pop,                       5 1           5       -       1
           perform operation, push result  4            

return 4 as the answer