题目：

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：

设置两个指针，一个从左向右，一个从右向左遍历。遇到非字母数字跳过，否则比较两指针的数值是否相等。

代码：

class Solution {
public:
    bool isPalindrome(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int i = 0;
        int j = s.size()-1;
        
        while(i<j)
        {
            while(!isAlpha(s[i]))
            {
                i++;
                if(i>=s.size())
                {
                    break;
                }
                
            };
            
            while(!isAlpha(s[j]))
            {
                j--;
                if(j<0)
                {
                    break;
                }
                
            };
            if(i<j&&!sameAlpha(s[i++],s[j--]))
            {
                return false;
            }
        };
        
        return true;
    }
    
    bool isAlpha(char c)
    {
        if(c - 'a' >= 0 && c - 'a' < 26)
        {
            return true;
        }
        else if(c - 'A' >= 0 && c - 'A' < 26)
        {
            return true;
        }
        else if(c >= '0' && c <= '9')
        {
            return true;
        }
        else
        {
            return false;
        }
    }
    
    bool sameAlpha(char c1, char c2)
    {
        if(c1 == c2)
        {
            return true;
        }
        else if(c1 - 'A' == c2 - 'a')
        {
            return true;
        }
        else if(c1 - 'a' == c2 - 'A')
        {
            return true;
        }
        else
        {
            return false;
        }
    }
};