125. Valid Palindrome

Description

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

For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.

Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

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

Solution

Two-pointer, O(n), S(1)

class Solution {
    public boolean isPalindrome(String s) {
        if (s == null) {
            return false;
        }
        
        for (int i = 0, j = s.length() - 1; i < j; ++i, --j) {
            while (i < j && !isValid(s, i)) {
                ++i;
            }
            
            while (i < j && !isValid(s, j)) {
                --j;
            }
            
            if (i < j && Character.toLowerCase(s.charAt(i)) 
                != Character.toLowerCase(s.charAt(j))) {
                return false;
            }
        }
        
        return true;
    }
    
    private boolean isValid(String s, int i) {
        char c = Character.toLowerCase(s.charAt(i));
        return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'z');
    }
}