High Sliding Window Question 153 of 224

How do you find the longest substring with at most k distinct characters?

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

Without indexScan every row

With indexJump to keys

CostWrites slower

Grow a right pointer with a frequency map

Why interviewers care about Sliding Window:

This is a process

question about Sliding Window.

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
Grow a right pointer
with a frequency map
2
while the map has
more than k keys, decrement s[left] and drop it at zero, then advance left.
3
Each index moves at
most twice: O(n) time and O(k) space.
4
For k=2 this is
the classic fruit-into-baskets problem.
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

“Each index moves at most twice: O(n) time and O(k) space.”

Break into beats

Eachindexmovesatmosttwice

Speaking order

2987408337471632900

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

Grow a right pointer with a frequency map while the map has more than k keys, decrement s[left] and drop it at zero, then advance left.