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.

 

这题直接在头尾设置两个指针，同时向中间扫面，每扫到一个数字或字母就停下来，比较是否相等，若不等就不是回文。代码如下：

 

class Solution {
public:
    bool isPalindrome(string s) {
        if( s.empty() ) return true;
        int left = 0;
        int right = s.length() - 1;
        while( left < right ) {
            while( left < right && !isalnum(s[left]) ) ++left;  //从左往右寻找alpha num 
            while( left < right && !isalnum(s[right]) ) --right;    //从右往左寻找alpha num
            if( left < right && tolower(s[left]) != tolower(s[right]) ) return false;   //在忽略大小写的情况下，如果不等，则不是回文
            ++left;
            --right;
        }
        return true;
    }
};