> For the complete documentation index, see [llms.txt](https://joshualbarb.gitbook.io/leetcode-problems/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://joshualbarb.gitbook.io/leetcode-problems/strings/string-simulation/valid-palindrome-i.md).

# Valid Palindrome I

String Simulation/Two Pointers

## Problem

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

**Note:** For the purpose of this problem, we define empty string as valid palindrome.

{% hint style="info" %}
For example:

```
Input: "A man, a plan, a canal: Panama"
Output: true
```

```
Input: "race a car"
Output: false
```

{% endhint %}

### Thought Process

* A **palindrome** is the same word spelt forwards and backwards

* We have to convert uppercase letters to lowercase and disregard spaces and special characters 

## Solution

```
class Solution:
    def isPalindrome(self, s: str) -> bool:
        l = 0
        r = len(s) - 1
        
        while l < r:
            while l < r and not s[l].isalnum():
                l+=1
            while l < r and not s[r].isalnum():
                r-=1
            if s[l].lower() != s[r].lower():
                return False
            l+=1
            r-=1
        return True
```

## Key Points

* Two pointers approach: one starting from the beginning and one at the end

* Check whether each element is alphanumeric only, if so then convert to lowercase.

* If either element in not alphanumeric, either increment or decrement either pointer, respectively&#x20;

## Time Complexity

* **Time:** $$O(n)$$ where n is the number of characters in the string
* **Space:** $$O(1)$$&#x20;