Parse a URL query string — interview question

Parse a URL query string into an object, the way a web framework fills req.query.

Rules:

An optional leading ? is ignored. Pairs are separated by &; empty segments are skipped.
In each segment, the text before the first = is the key and the rest is the value. A segment with no = has the value "".
Decode keys and values: + means a space, and %XX escapes are UTF-8 bytes (decode as decodeURIComponent does).
A key that appears once maps to a string. A key that appears more than once maps to an array of all its values, in order.
Keys appear in the output in the order they are first seen.

Constraints

The line is at most 5000 characters and is correctly encoded.
Decoded keys are non-empty and made of letters only.

Input and output

Input: One line: the query string.
Output: One line: the result as compact JSON.

Examples

Input 1

?course=btech&branch=cse&branch=ece&city=New+Delhi

Output

{"course":"btech","branch":["cse","ece"],"city":"New Delhi"}

Repeated keys become arrays; + means a space.

Input 2

q=fees%20%26%20placements&page=2&debug

Output

{"q":"fees & placements","page":"2","debug":""}

%20 is a space and %26 is &; a key without = gets an empty value.

Show hints

Replace + with a space before percent-decoding, not after (an encoded %2B must stay a plus).
Split each segment on the first = only.

Show the explanation

Split on &, skip empty pieces, and split each piece at the first = (indexOf + slice, not split('='), because values may contain encoded or literal =).

Decode in the right order: turn + into spaces first, then decodeURIComponent. Doing it the other way would turn an encoded plus (%2B) into a space.

When a key repeats, convert the stored string into an array on the second occurrence and push after that. Use an object without a prototype (Object.create(null)) or a Map, so keys such as constructor cannot clash with inherited properties — a real security concern known as prototype pollution.

Node's built-in URLSearchParams does the parsing (new URLSearchParams(s).getAll(key)); interviewers ask for a hand-written version to test string handling.

Original UniCircle question. Last reviewed 22 Oct 2026.