Moderate Stack Question 97 of 224

How do you decode a string with nested k[encoded] patterns?

DSA interview set · Speak this in 60–90 seconds · Faridabad & Delhi NCR

PICTURE THIS: STACK VS QUEUE

StackLIFOlast in, first out

QueueFIFOfirst in, first out

Simple meaning

Use a stack of (previous_string, multiplier) pairs: on '[', push the current buffer and k, on ']', pop and repeat the inner buffer.

WHY — Stack instead of guessing?

Why interviewers care about Stack:

This is a process

question about Stack.

Panels listen for order,

trade-offs, and what you would actually do on a DSA project - not buzzwords.

Stay structured

Name the idea, why it exists, then one short example.

Close cleanly

End with when you use it and one common pitfall.

STEPS — What happens step by step?

Before you speak the answer, walk the interviewer through these steps:

1
Use a stack of
(previous_string, multiplier) pairs: on '[', push the current buffer and k, on ']', pop and repeat the inner buffer.
2
Digits build k and
letters append to the current buffer.
3
Time is O(output length)
and space is O(output length), which can be much larger than the input.
4
Give an example
One tiny concrete case you can say aloud.
5
Common mistake
What juniors usually get wrong.
6
Close
When you pick this over the alternative.

EXAMPLE — See it in action

Here's a short line you can speak, broken into clear beats:

Say this line

“Digits build k and letters append to the current buffer.”

Break into beats

Digitsbuildkandlettersappend

Speaking order

2987408337471632900

Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.

Key takeaway

Use a stack of (previous_string, multiplier) pairs: on '[', push the current buffer and k, on ']', pop and repeat the inner buffer. Digits build k and letters append to the current buffer.