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functions.py$.$ Test it from

t$06.$py$.$

The function must use a single stack to do its work.

In a postfix expression, operators follow operands. Thus the infix expression:

$12-5$

is written as postfix expression:

$125$

which evaluates to $7.$

In postfix expressions as the operations are performed in the order that they appear. The expression:

$45+12**23**-$

is equivalent to $(4+5)**12-2**3$ and evaluates to $102.$

The algorithm for evaluation postfix expressions is:

Create a stack to store operands $($i$.$e$.$ values$)$

Walk through the expression string element by element:

If the element is a number, push it into the stack

When the expression is finished, the number on the top of the stack is the final answer. The stack should be empty after this number is popped.

You do not have to do any error handling on poorly written postfix expressions. A valid postfix expression has at least one number in it$.$