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.
思路：题目不难，头尾向中间逼近即可，代码如下：

class Solution {public:    bool isPalindrome(string s) {        int i = 0;        int j = s.length() - 1;        if(j < 1)            return true;        while(i <= j){            if(!isalnum (s[i]))                i++;            else if(!isalnum (s[j]))                j--;            else{                int tmp = abs(s[i] - s[j]);                if(tmp == 0 || tmp == 32){                    i++;                    j--;                }                else                    return false;            }        }        return true;    }};