leetcode：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.

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：