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Commit 085d4e1

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Add valid palindrome solution
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‎.gitignore‎

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

‎125_Valid_Palindrome/README.md‎

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

‎125_Valid_Palindrome/solution.py‎

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

‎README.md‎

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# leetcode-interviews
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Leetcode tasks from my interviews
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# LeetCode Interview Solutions
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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.
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## 📁 Repository Structure
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Solutions are organized by problem name. Each solution includes:
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- Implementation in one or more programming languages
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- Brief problem description
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- Complexity analysis
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- Key insights/approach
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## 🔍 Problem Format
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Each problem directory contains:
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* `README.md` with:
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* Problem statement
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* Example I/O
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* Approach explanation
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* Time & space complexity
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* Solution files (`solution.{ext}`)

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