Update prefixexpression.cpp

Prefix Expression Evaluation using stack and queue
Problem Statement : 
You are given a prefix expression. Write a program to evaluate it.

Input sample:
The first argument will be an input file with one prefix expression per line. e.g.
* + 2 3 4
Your program has to read this and insert it into any data structure you like. Traverse that data structure and evaluate the prefix expression. Each token is delimited by a whitespace. You may assume that the only valid operators appearing in test data are '+','*' and '/'

Output sample:
Print to stdout, the output of the prefix expression, one per line. e.g.
20