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.
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.
Stats
| Frequency | 5 |
| Difficulty | 2 |
| Adjusted Difficulty | 1 |
| Time to use | -------- |
Ratings/Color = 1(white) 2(lime) 3(yellow) 4/5(red)
Analysis
Wow, it’s frequency 5!
This is a very classic and easy question.
Code
public boolean isPalindrome(String s) {
int len = s.length();
s = s.toLowerCase();
int left = 0, right = len - 1;
while (left < right) {
while (left < len && !isAlphanumeric(s.charAt(left)))
left ++;
while (right >= 0 && ! isAlphanumeric(s.charAt(right)))
right --;
if (left == len && right == -1) return true;
if (left == len || right == -1) return false;
if (s.charAt(left) != s.charAt(right))
return false;
left ++;
right --;
}
return true;
}
private boolean isAlphanumeric(char c) {
return ('a' <= c && c <= 'z')
|| ('0' <= c && c <= '9');
}