Valid Palindrome

Approach

1. Normalize the string:
   • convert to lowercase
   • remove non-alphanumeric characters
2. Initialize two pointers:
   • l at the start
   • r at the end
3. Compare mirrored characters while l < r:
   • if they differ, return false
   • if they match, move inward (l++, r--)
4. If the loop completes, return true.

Implementation

export function isPalindrome(s: string): boolean {
    const normalized = s.toLowerCase().replace(/[^a-z0-9]/g, "");
    if (normalized.length <= 1) return true;
 
    let l = 0;
    let r = normalized.length - 1;
 
    while (l < r) {
        if (normalized[l] !== normalized[r]) {
            return false;
        }
 
        l++;
        r--;
    }
 
    return true;
}

Complexity

• Time O(n): We scan each character once while normalizing and at most once more while comparing mirrored pairs.
• Space O(n): The normalized string may store up to all characters from the input.