[LeetCode 125] Valid Palindrome

https://leetcode.com/problems/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.

这题比较常规，思路也很简单，就是 two pointers。一根指针指向字符串的开始位置，另一根指针指向字符串的结束位置。当中使用 while 循环忽略掉 non-alphanumeric 字符即可。来看下面代码：

class Solution {
public:
    bool isPalindrome(string s) {
        string::const_iterator left = s.cbegin();
        string::const_iterator right = s.cend() - 1;
        
        while (left < right) {
            
            // 忽略掉 non-alphanumeric 字符 
            while (left < right && !isalnum(*left)) {
                left++;
            }
            while (left < right && !isalnum(*right)) {
                right--;
            }
            
            if (tolower(*left) != tolower(*right)) {
                return false;
            } else {
                left++;
                right--;
            }
        }
        
        return true;
        
    }
};