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How to Solve Decode String Problem
Master the Decode String LeetCode problem with undetectable real-time assistance. Get instant solutions and explanations during your coding interviews.
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Start Free for WindowsMedium#394
LeetCode ProblemDecode String
Given an encoded string, return its decoded string. The encoding rule is: k[encoded_string], where the encoded_string 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.
StringStackRecursion
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Problem Breakdown
Understanding the Decode String Problem
Let's break down this LeetCode problem and understand what makes it challenging in interview settings.
Problem Statement
Given an encoded string, return its decoded string. The encoding rule is: k[encoded_string], where the encoded_string 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.
MediumProblem #394
LeetCodeDecode String
Related Topics
StringStackRecursion
How Phantom Code Helps
Get real-time assistance for Decode String problems during coding interviews. Phantom Code provides instant solutions and explanations.
Examples
# 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].
How Phantom Code Helps with LeetCode Problems
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Solve Decode String — Given an encoded string, return its decoded string. The encoding rule is: k[enco...
Here's the optimal approach using String:
def solve(input):
# Optimal O(n) solution
return result
Time: O(n) | Space: O(n)
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Undetectability Checklist
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Platform Compatibility
Zoom
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Google Meet
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HackerRank
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CodeSignal
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CoderPad
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Teams
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Undetectability and technical details
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Frequently Asked Questions
Common questions about solving Decode String and using Phantom Code during coding interviews.
How does Phantom Code help me solve coding problems like Decode String during interviews?
Phantom Code generates complete solutions instantly with proper complexity analysis, letting you focus on explaining your approach and demonstrating problem-solving skills rather than getting stuck on implementation details during high-pressure situations.
What if the interviewer asks follow-up questions or modifications to problems like Decode String?
Phantom Code adapts in real-time. Screenshot the modified problem or use audio mode to capture the interviewer's follow-up, and get an updated solution within seconds — including edge cases and optimizations.
Can Phantom Code help me communicate my solution for problems like Decode String?
Yes. Phantom Code provides step-by-step approach explanations with 3 progressive thoughts, so you can walk through your solution naturally. It also provides time and space complexity analysis to discuss trade-offs.
How does Phantom Code assist with different programming languages for Decode String type problems?
Phantom Code supports 11 programming languages including Python, Java, C++, JavaScript, TypeScript, Go, Rust, Ruby, Swift, Kotlin, and C#. Set your preferred language and get idiomatic solutions.
What coding mistakes does Phantom Code help me avoid during Decode String interviews?
Phantom Code catches common mistakes like off-by-one errors, missing edge cases, incorrect base conditions, and suboptimal approaches. It provides clean, well-structured code that handles all edge cases.
Is Phantom Code detectable when solving problems like Decode String during live interviews?
No. Phantom Code runs as an invisible desktop overlay that doesn't appear in screen shares, recordings, or proctoring software. It works with Zoom, HackerRank, CodeSignal, CoderPad, and all major platforms.
How does Phantom Code help when I'm struggling with Decode String style questions under pressure?
The AI provides structured guidance: first the approach (what algorithm/data structure to use and why), then the implementation with clean code, and finally complexity analysis. This helps you think clearly even under pressure.
Does Phantom Code work for the coding platforms used in Decode String interviews?
Yes. Phantom Code works with all major coding platforms including LeetCode, HackerRank, CodeSignal, CoderPad, HackerEarth, and any browser-based coding environment.
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