Add valid palindrome solution

Author: onixlas

.gitignore

@@ -165,7 +165,7 @@ cython_debug/
 #  be found at https://github.com/github/gitignore/blob/main/Global/JetBrains.gitignore
 #  and can be added to the global gitignore or merged into this file.  For a more nuclear
 #  option (not recommended) you can uncomment the following to ignore the entire idea folder.
-#.idea/
+.idea/
 
 # Abstra
 # Abstra is an AI-powered process automation framework.

125_Valid_Palindrome/README.md

@@ -0,0 +1,70 @@
+# Valid Palindrome
+
+## Problem Description
+A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.
+
+Given a string `s`, return `True` if it is a palindrome, or `False` otherwise.
+
+**Example 1:**  
+Input: `s = "A man, a plan, a canal: Panama"`  
+Output: `true`  
+Explanation: "amanaplanacanalpanama" is a valid palindrome.
+
+**Example 2:**  
+Input: `s = "race a car"`  
+Output: `false`  
+Explanation: "raceacar" is not a palindrome.
+
+## Solutions
+
+### Solution 1: String Comparison
+
+```python
+def is_palindrome(s: str) -> bool:
+    s = ''.join([char.lower() for char in s if char.isalnum()])
+    return s == s[::-1]
+```
+
+* **Time Complexity:** O(n)
+  * Linear pass to filter/convert characters
+  * O(n) string reversal comparison
+
+* **Space Complexity:** O(n)
+  * Stores cleaned version of input string
+
+### Solution 2: Two Pointers (In-Place)
+
+```pethon
+def is_palindrome2(s: str) -> bool:
+    left = 0
+    right = len(s) - 1
+
+    while left < right:
+        while (left < right) and not s[left].isalnum():
+            left += 1
+        while (left < right) and not s[right].isalnum():
+            right -= 1
+        if s[left].lower() != s[right].lower():
+            return False
+
+        left += 1
+        right -= 1
+
+    return True
+```
+
+* **Time Complexity:** O(n)
+  * Each character processed at most twice (once per pointer)
+* **Space Complexity:** O(1)
+  * Operates directly on input with pointer variables
+
+### Implementation Notes for Two-Pointer Solution
+
+* **Case Conversion is Critical**
+
+    Always convert compared characters to lowercase using .lower() before comparison.
+    Example: 'A' vs 'a' should be treated as equal.
+* **Update Pointers After Comparison**
+
+    Remember to increment left and decrement right at the end of the loop.
+    Failure to update pointers causes infinite loops.

125_Valid_Palindrome/solution.py

@@ -0,0 +1,42 @@
+def is_palindrome(s: str) -> bool:
+    """
+    First solution. Time complexity is O(n), memory complexity is O(n).
+    Creates a cleaned string (lowercase alphanumeric characters) and checks if it reads the same forwards and backwards.
+    :param s: Input string to check
+    :return: True if the string is a valid palindrome, False otherwise
+    """
+    s = ''.join([char.lower() for char in s if char.isalnum()])
+    return s == s[::-1]
+
+assert is_palindrome("A man, a plan, a canal: Panama") == True, 'Test 1 failed'
+assert is_palindrome("race a car") == False, 'Test 2 failed'
+assert is_palindrome(" ") == True, 'Test 3 failed'
+
+def is_palindrome2(s: str) -> bool:
+    """
+    Second solution. Time complexity is O(n), memory complexity is O(1).
+    Uses two pointers to compare characters from both ends, skipping non-alphanumeric characters.
+    Case-insensitive comparison is performed for alphanumeric characters.
+
+    :param s: Input string to check
+    :return: True if the string is a valid palindrome, False otherwise
+    """
+    left = 0
+    right = len(s) - 1
+
+    while left < right:
+        while (left < right) and not s[left].isalnum():
+            left += 1
+        while (left < right) and not s[right].isalnum():
+            right -= 1
+        if s[left].lower() != s[right].lower():
+            return False
+
+        left += 1
+        right -= 1
+
+    return True
+
+assert is_palindrome2("A man, a plan, a canal: Panama") == True, 'Test 1 failed'
+assert is_palindrome2("race a car") == False, 'Test 2 failed'
+assert is_palindrome2(" ") == True, 'Test 3 failed'

README.md

@@ -1,2 +1,23 @@
-# leetcode-interviews
-Leetcode tasks from my interviews
+# LeetCode Interview Solutions
+
+A curated collection of algorithmic solutions to problems encountered during technical interviews at various companies. This repository serves as both a personal reference and a resource for others preparing for coding interviews.
+
+## 📁 Repository Structure
+
+Solutions are organized by problem name. Each solution includes:
+
+- Implementation in one or more programming languages
+- Brief problem description
+- Complexity analysis
+- Key insights/approach
+
+## 🔍 Problem Format
+
+Each problem directory contains:
+
+* `README.md` with:
+	* Problem statement
+	* Example I/O
+	* Approach explanation
+	* Time & space complexity
+* Solution files (`solution.{ext}`)