leetcode 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.

思路：双端扫描，有非字母数字就跳过。

class Solution {
public:
bool isPalindrome(string s) {
if (s.empty()) return true;
int l = 0;
int r = s.size() - 1;
while (l <= r){
while (l<=r&&!isalnum(s[l]))
l++;
while (l <= r&&!isalnum(s[r]))
r--;
if (l > r) break;
if (s[l] == s[r]||s[l]+32==s[r]||s[l]==s[r]+32){
l++;
r--;
}
else
return false;
}
return true;
}
};