Moderate DP Question 130 of 224

How do you count ways to decode a digit string mapped to A-Z?

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

IdeaDP

HowWhat happens inside

Why they askShows real use

dp[i] adds dp[i-1] if the last digit is 1-9 and dp[i-2] if the last two digits are 10-26.

Why interviewers care about DP:

This is a process

question about DP.

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.

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

1
dp[i] adds dp[i-1] if
the last digit is 1-9 and dp[i-2] if the last two digits are 10-26.
2
Time is O(n) and
space is O(1) with two rolling values.
3
Leading zeros block a
single-digit decode
4
Give an example
handle them explicitly.
5
Common mistake
What juniors usually get wrong.
6
Close
When you pick this over the alternative.

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

Say this line

“Time is O(n) and space is O(1) with two rolling values.”

Break into beats

TimeisOnandspace

Speaking order

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

dp[i] adds dp[i-1] if the last digit is 1-9 and dp[i-2] if the last two digits are 10-26. Time is O(n) and space is O(1) with two rolling values.