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.

分析：题意为 给出一个字符串，检查它是不是一个回文的情况。判断时只考虑字母数字的字符并且忽略大小写。

注意点 1、考虑字符串为空的情况，并将其作为回文的

          2、忽略大小写的差别，比较之前先将大小写转换为一致的

          3、非字母数字字符要跳过，将它滤除，得到有效字符串

          4、对有效字符串前后相应位置进行比较判断

代码如下：

class Solution {  public:      bool isStr(char &ch){          if(ch >= '0' && ch <= '9'){              return true;          } else if(ch >= 'a' && ch <= 'z'){              return true;          } else if(ch >= 'A' && ch <= 'Z'){             ch += 32;             return true;          }                     return false;      }            bool isPalindrome(string s) {          // Start typing your C/C++ solution below          // DO NOT write int main() function          int len = s.length();          if(len == 0){              return true;          }                    string str = "";          for(int i = 0; i < len; i++){   // remove illegal char, such as "?" "/" ...              if(isStr(s[i])){                  str += s[i];              }          }                    len = str.length();          int mid = (len + 1) / 2;          for(int i = 0; i < mid; i++){              if(str[i] != str[len - 1 - i]){     // check front and end char                  return false;              }          }          return true;      }  };

其他解法：