Decode String

Algorithms
Medium
Apple
72.2K views

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. Use a Stack.

Why Interviewers Ask This

Apple interviewers use the 'Decode String' problem to evaluate a candidate's ability to manage nested structures and state transitions. They specifically test proficiency with stack-based algorithms, which are fundamental for parsing complex data formats like JSON or code compilers. The question reveals how well you handle recursion logic iteratively and your capacity to debug edge cases involving multiple levels of nesting without relying on built-in parser functions.

How to Answer This Question

1. Clarify the input format immediately by asking about valid integers, empty brackets, or nested patterns to confirm constraints. 2. Propose a two-stack approach: one for numbers and one for strings, explaining that this separates the count from the content being repeated. 3. Walk through a concrete example like '3[a]2[bc]' step-by-step on a whiteboard, showing how you push current counts and pop them when closing brackets appear. 4. Implement the logic using a single pass loop, detailing how you accumulate digits into an integer variable and concatenate characters until a '[' is encountered. 5. Conclude by analyzing time complexity as O(N) where N is the total length of the decoded string, noting space complexity based on the maximum nesting depth, which demonstrates strong algorithmic thinking valued at Apple.

Key Points to Cover

  • Explicitly defining the two-stack strategy to separate counts from string segments
  • Demonstrating the iterative handling of nested brackets without recursion
  • Correctly accumulating multi-digit numbers before processing the bracket
  • Analyzing time complexity as O(N) relative to the final decoded string length
  • Handling edge cases like nested brackets or consecutive repetitions seamlessly

Sample Answer

To solve the Decode String problem efficiently, I would implement an iterative solution using stacks to handle the nested repetition logic. First, I initialize two stacks: one for storing the repeat counts (integers) and…

Common Mistakes to Avoid

  • Attempting to use recursion without considering stack overflow risks for deep nesting levels
  • Failing to correctly parse multi-digit numbers, treating each digit as a separate count
  • Overlooking the need to save the previous string context before processing a new bracket pair
  • Not validating the input structure, leading to errors on malformed encoded strings

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