学步园

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：

两个指针左右移动的同时比较字符是否相同。

题解：

class Solution {
public:
    bool isPalindrome(string s) {
        size_t len = s.size();
        
        if (len == 0)
            return true;

        int l = 0;
        int r = len - 1;
        while(l < r)
        {
            while(!isalnum(s[l]) && l < r) l++;
            while(!isalnum(s[r]) && l < r) r--;
            if (tolower(s[l]) != tolower(s[r]))
                return false;
                
            l++; r--;
        }
        return true;
    }
};