Скачать или смотреть LeetCode 1930 🔥 Unique Length-3 Palindromic Subsequences | Optimal Counting Trick + Easy Logic

In this video, we solve LeetCode 1930 – Unique Length-3 Palindromic Subsequences, a beautiful problem based on prefix–suffix character presence.

Code:
https://github.com/studyPlacement/Cod...

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A length-3 palindrome looks like:

a _ a

So we just need:

same start & end character,

and any middle character between their first and last occurrence.


⭐ Optimal Approach (O(26 * n)):

For each letter c from 'a' to 'z':

1. Find first occurrence of c.


2. Find last occurrence of c.


3. Count all distinct characters between these two indices.


4. Add them to the result.



This avoids generating subsequences and gives a fast, clean solution.

⭐ What you will learn:

How to count palindromic subsequences efficiently

Why brute force fails

Prefix–suffix character frequency logic

Counting distinct middle characters

Clean code + dry run

Complexity: O(26 × n)


Perfect problem for building strong string-processing and observation skills.

🚀 Topics Covered:

Strings

Palindromic subsequences

Counting distinct characters

Frequency arrays

Optimized scanning


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