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.

思路：从一个字符串中提取字母和数字忽略其它字符，构成一个新字符串，判断它是否是回文

class Solution {public:        bool isPalindrome(string s) {        string str="";        //s=tolower(s);        for(int i=0;i<s.size();i++)        {            if(s[i]>='a'&&s[i]<='z')                str+=s[i];            else if(s[i]>='A'&&s[i]<='Z')                str+=tolower(s[i]);            else if(s[i]>='0'&&s[i]<='9')                str+=s[i];            else                continue;        }        if(str.size()<=1)             return true;        int l=0;        int r=str.size()-1;        while(l<r)        {            if(str[l]==str[r])            {                l++;                r--;            }            else                 return false;        }        return true;            }};