125. Valid Palindrome

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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.

Example 1:

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

Example 2:

Input: "race a car"
Output: false

Solution

    public boolean isPalindrome(String s) {
        int headIndex = 0, tailIndex = s.length()-1;
        
        while(headIndex <= tailIndex){
            char chead = s.charAt(headIndex);
            char ctail = s.charAt(tailIndex);
            //Remeber Character.isLetterOrDigit(c)
            if(!Character.isLetterOrDigit(chead)){
                headIndex++;
            }else if (!Character.isLetterOrDigit(ctail)){
                tailIndex--;
            }else{
                //Remeber Character.toLowerCase(c)
                if(Character.toLowerCase(chead) != Character.toLowerCase(ctail)){
                    return false;
                }
                headIndex++;
                tailIndex--;
            }
        }
        return true;
    }