# Leetcode Valid Palindrome String

Question:

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

Q: What about an empty string? Is it a valid palindrome?
A: For the purpose of this problem, we define empty string as valid palindrome.

Solution:

O(n) runtime, O(1) space:

The idea is simple, have two pointers – one at the head while the other one at the tail. Move them towards each other until they meet while skipping non-alphanumeric characters

Consider the case where given string contains only non-alphanumeric characters. This is a valid palindrome because the empty string is also a valid palindrome

public boolean isPalindrome(String s) {
int i = 0, j = s.length() - 1;
while (i < j) {
while (i < j && !Character.isLetterOrDigit(s.charAt(i))) i++;
while (i < j && !Character.isLetterOrDigit(s.charAt(j))) j--;
if (Character.toLowerCase(s.charAt(i))
!= Character.toLowerCase(s.charAt(j))) {
return false;
}
i++; j--;
}
return true;
}