Decode String · Interview Problem

Back to problems

Leetcode

decode-string

Medium

Stacks

Recursion

String Processing

Decode String

Decode an encoded string where patterns of the form $k[encoded_string]$ repeat the enclosed substring $k$ times.

Acceptance 0%

Start Practice

Problem Statement

Core description from the backend problem record.

Description

Given an encoded string, return its decoded string.

The encoding rule is: $k[encoded_string]$ , where the

clike

encoded_string

inside the square brackets is being repeated exactly

k

times.

clike

$k$

is guaranteed to be a positive integer.

You may assume that the input string is always valid; no extra white spaces, square brackets are well-formed, and no invalid characters are present. The original data does not contain any digits and digits are only for those repeat numbers, $k$ . For example, there will not be an input like

clike

3a

clike

2[4]

The decoded string may be long, but it is guaranteed to fit in a 32-bit signed integer length.

Input Format

A single encoded string $s$ .

Output Format

Return the decoded string.

Constraints

The input string is valid.
Digits appear only as repeat counts $k$ .
The decoded result fits in a 32-bit signed integer length.

Examples

Sample cases returned by the problem API.

Example 1

Input

s = "3[a]2[bc]"

Output

"aaabcbc"

Explanation

clike

3[a]

becomes

clike

aaa

, and

clike

2[bc]

becomes

clike

bcbc

. Concatenating them gives

clike

aaabcbc

Example 2

Input

s = "3[a2[c]]"

Output

"accaccacc"

Explanation

Inner

clike

2[c]

becomes

clike

cc

, so

clike

3[a2[c]]

becomes

clike

3[acc]

which is

clike

accaccacc

Show 1 more example

Example 3

Input

s = "2[abc]3[cd]ef"

Output

"abcabccdcdcdef"

Explanation

clike

2[abc]

becomes

clike

abcabc

clike

3[cd]

becomes

clike

cdcdcd

, then append

clike

ef

Premium problem context

Unlock deeper context for this problem

Premium adds guided hints, editorial links, similar variants, discussion resources, and concept maps so you can understand why a problem matters, not just solve it once.

Guided hints

Editorial and discussion links

Concept map and variants

Track your progress