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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.

基本思路：

题目要求，判断一字符串，是否为符号，只考虑字母和数字，并且忽略大小写。

用 isalnum 判断是否为字母和数字；

用tolower将基本都作转换小写字母，从而在比较时忽略掉大小写。

设置一头一尾两个指针，向中间移动。

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此代码在leetcode上实际执行时间为14ms。

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

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