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.

思路：左右指针移动，首先转换成lowercase，然后判断是不是alphanumeric，如果不是，左移右移直到left>=right.

if(left>right) break; 这句话一定要加上，别忘了。

public class Solution {     public boolean isPalindrome(String s) {         if(s == null) return false;         int left = 0; int right = s.length()-1;         while(left<=right){             while(left<s.length() && !isvalid(s.charAt(left))){                 left++;             }                          while(right>=0 && !isvalid(s.charAt(right))){                 right--;             }             if(left>=right) break;                          if(changeToLowerCase(s.charAt(left)) != changeToLowerCase(s.charAt(right))){                 return false;             }                          left++;             right--;         }         return true;     }          public boolean isvalid(char c){         return ('0'<=c && c<='9') ||('a'<=c && c<='z') ||('A'<=c && c<='Z');     }          public char changeToLowerCase(char c){         if('a'<=c && c<='z') return c;         int diff = 'a'-'A';         return (char)(c+diff);     }      }