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 = 0, end = s.length() - 1;
        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;
    }
};