125 Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:

Input: "A man, a plan, a canal: Panama"
Output: true

Example 2:

Input: "race a car"
Output: false

这个题要小心全是符号的情况，还有就是其中有数字的情况。这里只是判断字母，遇到数字跳过

isalnum()函数可以判断一个字符是不是 字母或者数字

代码：

class Solution {
public:
    bool isPalindrome(string s) {
        if(s.length() == 0 || s.length() == 1)
            return true;
        transform(s.begin(),s.end(),s.begin(),towlower);
        int i = 0;
        int j = s.length()-1;
        while(i<j){         
            while(!isalnum(s[i]) && i<j)i++;
            while(!isalnum(s[j]) && i<j)j--;
            if(s[i]==s[j]){
                i++;
                j--;
            }else
                return false;          
        }
        return true;
    }
};