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：

将原字符串去掉非字母和数字，全部转化为小写，在判断回文

代码：

    bool isPalindrome(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        string t = "";
        for (int i = 0; i < s.length(); i++) {
            if (s[i] >= 'a' && s[i] <= 'z' || s[i] >= '0' && s[i] <= '9' || s[i] >= 'A' && s[i] <= 'Z') {
                if (s[i] >= 'A' && s[i] <= 'Z') {
                    t += (s[i] - 'A' + 'a');
                }
                else {
                    t += s[i];                    
                }
            }
        }
        if (t == "") {
            return true;
        }
        for (int i = 0; i < t.length() / 2; i++) {
            if (t[i] != t[t.length() - i - 1]) {
                return false;
            }
        }
        return true;
    }