Challenge: Prefix to Postfix Conversion

Problem statement

In this challenge, write a function postfix(prefix) that takes a prefix expression as prefix, converts it to a postfix expression using a stack, and then returns it.

• Prefix expression: An expression in which the operator appears before the operands.

• Postfix expression: An expression in which the operator appears after the operands.

For example, if we have an expression $A+B$, then its prefix expression would be $+AB$, and its postfix expression would be $AB+$.