# 125. Valid Palindrome

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.

``````class Solution {
public:
bool isPalindrome(string s) {
int left = 0, right = s.size() - 1 ;
while (left < right) {
if (!isAlphaNum(s[left])) ++left;
else if (!isAlphaNum(s[right])) --right;
else if ((s[left] + 32 - 'a') %32 != (s[right] + 32 - 'a') % 32) return false;
else {
++left; --right;
}
}
return true;
}
bool isAlphaNum(char &ch) {
if (ch >= 'a' && ch <= 'z') return true;
if (ch >= 'A' && ch <= 'Z') return true;
if (ch >= '0' && ch <= '9') return true;
return false;
}
};
``````

``````class Solution {
public:
bool isPalindrome(string s) {
int left = 0, right = s.size() - 1 ;
while (left < right) {
if (!isalnum(s[left])) ++left;
else if (!isalnum(s[right])) --right;
else if ((s[left] + 32 - 'a') %32 != (s[right] + 32 - 'a') % 32) return false;
else {
++left; --right;
}
}
return true;
}
};
``````

https://discuss.leetcode.com/topic/5581/here-s-a-clean-c-solution/2

https://discuss.leetcode.com/topic/25405/my-three-line-java-solution

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