Prefix to Postfix Conversion

The problem. Given a valid prefix expression (operator before its operands), convert it to postfix (operator after its operands). Operands are single letters or digits; operators are + - * / ^. For example, *+AB-CD becomes AB+CD-*.

Example

Input:  "*+AB-CD"        Output: "AB+CD-*"
Input:  "*-A/BC-/AKL"    Output: "ABC/-AK/L-*"
Input:  "A"              Output: "A"

Neither prefix nor postfix needs brackets, so no brackets appear anywhere. Each operator just has to move from before its two operands to after them — and its operands may themselves be whole expressions.