All ways are allowed.
Look at http://en.wikipedia.org/wiki/Reverse_Polish_notation
Converting from infix notation
Main article: Shunting-yard algorithm
Edsger Dijkstra invented the Shunting-yard algorithm to convert infix expressions to postfix (RPN), so named because its operation resembles that of a railroad shunting yard.
There are other ways of producing postfix expressions from infix notation. Most Operator-precedence parsers can be modified to produce postfix expressions; in particular, once an abstract syntax tree has been constructed, the corresponding postfix expression is given by a simple post-order traversal of that tree.
See: http://en.wikipedia.org/wiki/Tree_traversal
Traversal
Compared to linear data structures like linked lists and one dimensional arrays, which have only one logical means of traversal, tree structures can be traversed in many different ways.
Possible ways of passing binary tree:
There are 6 ways:
VLR - pre-order (Ćukasiewicz),
LVR - in-order,
LRV - post-order (RPN),
VRL,
RVL,
RLV,
where :
V - Visit node,
L - go Left,
R - goRight.
It is interesting to see mathematical expressions in all six ways.
-but, why I have not yet done - it's a question!
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for the question - I can't give a respond - it all depends on the implementation details