leetcode: Valid Palindrome

判断是否回文，注意包括数字和字母，忽略大小写

class Solution {public:    bool isPalindrome(string s) {        string tmp = "";        for( int i = 0; i < s.size(); ++i){            if(isalnum(s[i])){                if(isupper(s[i]))                    tmp += static_cast<char>(tolower(s[i]));                else                    tmp += s[i];            }        }        for(int i = 0, j = tmp.size() - 1; i < j; ++i, --j){            if( tmp[i] != tmp[j])                return false;        }        return true;    }};