leetcode题解：Valid Palindrome（判断回文）

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.

说明：

       1）注意两点：a、判断满足条件的字符  b、大写变小写（本题认为不区分大小写）

       2）字符串为空则true

实现：

class Solution {
public:
        bool isPalindrome(string s) {
            if(s.empty()) return true;
            int len=strlen(s.c_str());
            int i=0;
            int j=0;
            for (;i<len;i++)
            {
                if (isChar(s[i]))//去其他符合，若有大写则大写变小写
                {
                    s[i] = tolower(s[i]);//库函数
                    s[j++]=s[i];
                }
            }
            s[j]='';
            if (s[0]=='') return true;
            int len2=strlen(s.c_str());
            i=0;
            while(i<=len2-1-i) //判断是否回文
            {
                if(s[i]!=s[len2-1-i]) return false;
                i++;
            }
            return true;            
    }
    private:
        bool isChar(char t)//判断是否满足条件字符
        {
            if (('a' <= t&&t<='z')||('A' <= t&&t<='Z')||('0' <= t&&t<='9'))
           {
               return true;
           }
           else
               return false;           
        }
};