125 Valid Palindrome

链接：https://leetcode.com/problems/valid-palindrome/
问题描述：
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.

Hide Tags Two Pointers String
Hide Similar Problems (E) Palindrome Linked List

求一个字符串是不是回文字符串，只关注字母和数字，并且忽略字母的大小写。

class Solution {
public:
    bool isPalindrome(string s) {
        string str="",rstr;
        for(int i=0;i<s.size();i++)
        {
          if((s[i]>47&&s[i]<58)||(s[i]>64&&s[i]<91))
              str+=s[i];
          else if(s[i]>96&&s[i]<123)
              str+=s[i]-32;
        }
        rstr=str;
        reverse(str.begin(),str.end());
        return rstr==str;
    }
};