125. Valid Palindrome【easy】

125. Valid Palindrome【easy】

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.

解法一：

 class Solution {
 public:
     bool isPalindrome(string s) {
         int start = , end = s.length() - ;
         while (start <= end) {
             if (!isalnum(s[start])) {
                 start++;
             } else if (!isalnum(s[end])) {
                 end--;
             } else {
                 if (tolower(s[start++]) != tolower(s[end--])) {
                     return false;
                 }
             }
         }
         return true;
     }
 };