DSA interview question

Postfix to Prefix Conversion

Medium · Data structures · Topics: stack, string

The problem

A stack-machine assembler writes expressions in postfix (for example ABC/-AK/L-*), but a symbolic-math library it feeds requires prefix (*-A/BC-/AKL). Postfix is read naturally left to right: operands are pushed, and when an operator arrives its two operands are already on the stack, right on top, left below. Pop them in that order and push op + left + right.

Example: ABC/-AK/L-* → *-A/BC-/AKL.

Function signature

string postfixToPrefix(const string& post);  // single-character operands; binary operators

Test cases

Input	Output	Explanation
`ABC/-AK/L-*`	`*-A/BC-/AKL`
`ab+`	`+ab`
`ab+cd-*`	`*+ab-cd`
`ab+c-`	`-+abc`
`a`	`a`

(Confirm: the postfix expression is valid; operands are single characters; all operators are binary.)

How interviewers phrase it

"Convert a postfix expression to prefix" (GeeksforGeeks "Postfix to Prefix Conversion").

Clarifying questions to ask

Are all operators binary? (Yes.)
Single-character operands?
Is the input valid? (Otherwise check the stack sizes.)

Follow-ups to expect

"Why left to right here?"
"Which pop is which?"
"How do you detect invalid input?"
"How does this relate to evaluating postfix?"
"Complexity?"

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