DSA interview question

Prefix to Postfix Conversion

Medium · Data structures · Topics: stack, string

The problem

A compiler pipeline receives expressions in prefix form from one tool and must hand them to a stack-based evaluator that expects postfix. Both notations are parenthesis-free and unambiguous, so no precedence rules are involved. Only the operator placement moves. Scan the prefix string right to left: push operands; for an operator, pop the left operand (top) and the right operand, and push left + right + op.

Example: *-A/BC-/AKL → ABC/-AK/L-*.

Function signature

string prefixToPostfix(const string& pre);  // single-character operands; binary operators

Test cases

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

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

How interviewers phrase it

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

Clarifying questions to ask

Only binary operators? (Yes.)
Single-character tokens? (Yes; otherwise tokenise first.)
Is the input guaranteed valid?

Follow-ups to expect

"Why right to left?"
"Which pop is the left operand?"
"Why is no precedence needed?"
"Can you do it without strings?"
"Complexity?"

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