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.

题目标签：String， Two Pointers

　　题目给了我们一个string s， 让我们判断它是不是 palindorme。

　　对于s， 我们只需要比较 字母 和 数字。

设置一个 left = 0， right = s.length() -1，依次比较，其中遇到任何其他 char 就跳过，具体看code。

　　一开始不知道有 Character.isLetterOrDigit() ，还分别用了 letter 和 digit - -

Java Solution:

Runtime beats 80.01%

完成日期：03/07/2017

关键词：two pointers

关键点：skip all non-alphanumeric chars

 class Solution

 {

     public boolean isPalindrome(String s)

     {

         // use two pointers

         int left = 0;

         int right = s.length() - 1;

         char [] char_arr = s.toCharArray();

         while(left < right)

         {

             while(left < right && !Character.isLetterOrDigit(char_arr[left])) // if char is not letter or digit

                 left++;

             while(left < right && !Character.isLetterOrDigit(char_arr[right])) // if char is not letter or digit

                 right--;

             if(Character.toUpperCase(char_arr[left]) != Character.toUpperCase(char_arr[right]))

                 return false;

             left++;

             right--;

         }

         return true;

     }

 }

参考资料：n/a

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