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;
        }
    }
}