Create README.md

Author: lyushher

day-28/README.md

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+# Day 28
+
+## ⭐️ Longest Common Prefix – 28.1
+### 🔗 Problem
+[LeetCode #14 – Longest Common Prefix](https://leetcode.com/problems/longest-common-prefix/)
+
+### 🧠 Core Idea
+Choose the first word as a baseline and compare its characters across all other words, stopping when a mismatch is found or one of the words ends.
+The substring up to that point is the longest common prefix.
+This ensures we only check as far as characters match, giving an efficient early termination strategy.
+
+### 📊 Example
+Input: strs = \["flower","flow","flight"]
+
+Output: `"fl"`
+
+### ⏱️ Complexity
+- Time: O(S) – where S is the total number of characters across all strings (early exit possible)
+
+- Space: O(1) – Constant space usage
+
+👉 See full code in [longest_common_prefix.py](https://github.com/lyushher/LeetCode-Python-Easy-DSA/blob/main/day-28/longest_common_prefix.py)
+
+---
+
+## ⭐️ Keyboard Row – 28.2
+### 🔗 Problem
+[LeetCode #500 – Keyboard Row](https://leetcode.com/problems/keyboard-row/)
+
+### 🧠 Core Idea
+Each keyboard row can be represented as a set of letters. 
+For each word, convert its letters into a set and check if it is a subset of any row set.
+If yes, the word can be typed using letters from one row only.
+
+### 📊 Example
+Input: words = \["Hello","Alaska","Dad","Peace"]
+
+Output: \["Alaska","Dad"]
+
+### ⏱️ Complexity
+- Time: O(n·k) – n = number of words, k = average length of each word
+
+- Space: O(1) – Only three row sets used
+
+👉 See full code in [keyboard_row.py](https://github.com/lyushher/LeetCode-Python-Easy-DSA/blob/main/day-28/keyboard_row.py)