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');
}
}
