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.

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解题思路：

使用两个指针，一个从左到右，一个从右到左，当左指针值大于右指针时，说明String 遍历结束。注意：alphanumeric 包括 a-zA-Z0-9，一定要看清题意。

因为忽略大小写，所以就统一为小写。

同时面试时要问清楚当String is empty时，是否是valid palindrome.

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Java code:

public boolean isPalindrome(String s) {        if(s.length() == 0){            return true;        }        int p1 = 0, p2 = s.length()-1;  //p1: left to right p2: right to left        s= s.toLowerCase();        while(p1 <= p2){            char left = s.charAt(p1);            char right = s.charAt(p2);            if(!((left >='a' && left <= 'z') || (left >= '0' && left <= '9'))){                p1++;                continue;            }            if(!((right >='a' && right <= 'z') || (right >= '0' && right <= '9'))){                p2--;                continue;            }            if(left != right){                return false;            }else{                p1++;                p2--;            }        }        return true;    }

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Reference:

1. http://www.cnblogs.com/springfor/p/3884156.html