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.
去掉 non-alphanumeric 的字符，然后忽略大小写判断是不是回文串。
two pointers
class Solution { public: bool isPalindrome(string s) { string::const_iterator left = s.cbegin(); string::const_iterator right = s.cend() - 1; while (left < right) { while (left < right && !isalnum(*left)) left++; while (right > left && !isalnum(*right)) right--; if (tolower(*left) != tolower(*right)) return false; else { left++; right--; } } return true; } };
原文：http://www.cnblogs.com/pk28/p/7230090.html