LeetCode 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.

题意解读：判断一个string是否为palindrome（回文），即该string从头至尾与从尾至头的字符出现顺序是否一样如“abccba”。

<pre name="code" class="cpp">bool isPalindrome(string s) {                        //戴的方法                ::transform(s.begin() , s.end() , s.begin() , ::tolower);        string::iterator left=s.begin() , right=prev(s.end());        while(left<right){            if(!isalnum(*left)){                ++left;            }else if(!isalnum(*right)){                --right;            }else if(*left==*right){                ++left;                --right;            }else{                return false;            }        }        return true;                                        //我的方法        ::transform(s.begin() , s.end() , s.begin() , ::tolower);        string::iterator left = s.begin() , right = prev(s.end());        while(left<right){            while(!isalnum(*left)){                ++left;                if(left==s.end()){        //如果left==s.end(),则说明该字符串不为空且，一直没有出现字母和数字如“。。。。。。”                    return true;                }            }            while(!isalnum(*right)){                --right;            }            if(*left==*right){                ++left;                --right;            }else{                return false;            }        }        return true;    }