Uppercase + Lowercase = Special? | Counting Special Letters I | LeetCode 3120

In this video, we solve "Counting Special Letters I" (LeetCode 3120) and uncover the simple observation that makes the problem easy to solve. At first glance, it looks like a character frequency problem... but the actual trick is checking whether BOTH lowercase and uppercase versions of the same letter exist. We'll cover: • Problem understanding • Character presence tracking • Lowercase and uppercase matching • Hashing approach • Optimized solution • Clean interview-ready implementation -------------------------------------------------- Key Insight: A letter is SPECIAL if: Both of these exist: a and A ✅ b and B ✅ But: a only ❌ A only ❌ For every character, we simply check whether its lowercase and uppercase versions are present. -------------------------------------------------- Example: Input: "aaAbBc" Present: a and A ✅ b and B ✅ c only ❌ Answer: 2 -------------------------------------------------- Time Complexity: O(n) Space Complexity: O(1) -------------------------------------------------- Source Code: GitHub: https://github.com/PalDPathak404/leet... -------------------------------------------------- Connect with me: LinkedIn: https://linkedin/in/paldpathak GitHub: https://github.com/PalDPathak404 Twitter/X: https://x.com/PalDPathak Instagram:   / pal_pathak.911   Portfolio: https://pal-pathak-sigma.vercel.app -------------------------------------------------- Topics Covered: Strings Hashing Character Frequency Arrays Simulation DSA Problem Solving LeetCode -------------------------------------------------- Timestamps: 00:00 Introduction 00:32 Problem Explanation 02:18 Bruteforce Approach 09:46 Optimised Approach 19:17 Outro -------------------------------------------------- #leetcode3120 #leetcode #strings #hashing #characters #arrays #dsa #algorithms #cplusplus #cpp #coding #programming #competitiveprogramming #softwareengineering #problemsolving #codependency