125. 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.

问题：判断一个字符串是否是回文字符串，只考虑字母和数字 忽略空格标点大小写等

思想：指针i 从前开始 指针j总后开始 向中间移动 ,当字符串为空串或者只有一个字符时它是回文的

   public boolean isPalindrome(String s) {        if(s==null||s.length()==1) return true;                int i=0;        int j=s.length()-1;        while(i<=j){        char left=s.charAt(i);        char right=s.charAt(j);        if(!Character.isAlphabetic(left)&&!Character.isDigit(left)){        i++;        }        else if(!Character.isAlphabetic(right)&&!Character.isDigit(right)){        j--;        }        else{        //System.out.println("left="+left+"\tright="+right);        left=Character.toLowerCase(left);        right=Character.toLowerCase(right);        if(left!=right) return false;        i++;j--;        }                }        return true;    }