Valid Palindrome

https://oj.leetcode.com/problems/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.

解题思路：

这题非常简单了。两个指针，一个从头，一个从结尾，往中间靠，直到他们相等。遇到非数字和字母的，不比较，跳过。遇到一个不相同的，就返回false。

要注意首先将整个字符串全变为大写或小写。其实，Character这个类有一个isLetterOrDigit(‘x’)的方法。

还要总结的是，||和&&在一起的时候，优先&&，然后||。

public class Solution {

    public boolean isPalindrome(String s) {

        if(s.length() == 0){

            return true;

        }else{

            s = s.toUpperCase();

            int right = s.length() - 1;

            int left = 0;

            while(left < right){

                if(!(s.charAt(left) >= 'A' && s.charAt(left) <= 'Z' || s.charAt(left) >= 'a' && s.charAt(left) <= 'z' || 

                s.charAt(left) >= '0' && s.charAt(left) <= '9')){

                    left++;

                    continue;

                }

                if(!(s.charAt(right) >= 'A' && s.charAt(right) <= 'Z' || s.charAt(right) >= 'a' && s.charAt(right) <= 'z' || 

                s.charAt(right) >= '0' && s.charAt(right) <= '9')){

                    right--;

                    continue;

                }

                if(s.charAt(left) != s.charAt(right)){

                    return false;

                }

                left++;

                right--;

            }

            return true;

        }

    }

}