125. 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.

翻译：有效的回文

给定一个字符串，确定这个字符串是否为回文，只考察字母数字,字符忽略，并且忽略大小写

谁的例子说实在的，感觉是错的，意思理解就好。多了一步，需要转化大小写进行统一。

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