Decode String
Decode String is a Medium Data Structures and Algorithms interview problem you can solve, run and submit on Thita.ai. It belongs to the Stack Patterns pattern, in the Simulation / Backtracking Helper subpattern.
Problem statement
Given an encoded string, return its decoded string.
The encoding rule is: k[encodedstring], where the encodedstring inside the square brackets is being repeated exactly k times. Note that k is guaranteed to be a positive integer.
You may assume that the input string is always valid; there are no extra white spaces, square brackets are well-formed, etc. Furthermore, you may assume that the original data does not contain any digits and that digits are only for those repeat numbers, k. For example, there will not be input like 3a or 2[4].
The test cases are generated so that the length of the output will never exceed 105.
Example 1:
Input: s = "3[a]2[bc]" Output: "aaabcbc"
Example 2:
Input: s = "3[a2[c]]" Output: "accaccacc"
Example 3:
Input: s = "2[abc]3[cd]ef" Output: "abcabccdcdcdef"
Constraints:
- 1 <= s.length <= 30
- s consists of lowercase English letters, digits, and square brackets '[]'.
- s is guaranteed to be a valid input.
- All the integers in s are in the range [1, 300].
Topics and companies
Topics: String, Stack, Recursion.
Reported in interviews at Bloomberg, Google, Amazon, Oracle, ByteDance, Apple, Walmart Labs, VMware, eBay, Cisco.
How to practise Decode String on Thita.ai
Starter code is provided in C, C#, C++, Go, Java, JavaScript and Python. Your solution runs against 5 test cases for this problem, with the AI coach available for a hint when you are stuck rather than a finished answer. The reference solution runs in O(n) time and O(n) space.
Open Decode String in the code editor, or read the Decode String editorial for a worked solution with its approach and complexity analysis.
Where Decode String sits in the DSA pattern sheet
- Stack Patterns pattern guide
- Simulation / Backtracking Helper subpattern
- The full DSA patterns sheet
- All coding practice problems
Problems related to Decode String
Other problems that use the same Simulation / Backtracking Helper and Stack Patterns techniques:
- Asteroid Collision — Medium, same subpattern
- Simplify Path — Medium, same subpattern
- Basic Calculator — Hard, same pattern
- Basic Calculator II — Medium, same pattern
- Basic Calculator III — Hard, same pattern
- Daily Temperatures — Medium, same pattern