Valid Palindrome

https://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.

题意：检测字符串是否为回文，只考虑数字和字母，空字符串视为回文

思路：从两边往中间扫描，发现非字母和数字则跳过，发现不相同直接返回false，循环结束则返回true

实现：

public class Solution {
      public boolean isPalindrome(String s ) {
           char [] a = s .toCharArray();
           int d = 'a' - 'A' ;//大写字母与小写字母的差值
           boolean flag = false; //检测是否出现过字母数字，若是没有出现且循环指向同一位置，则表示字符串空，则返回true
           for (int i = 0, j = s .length() - 1; i <= j;) {
               if (!flag && i == j &&!Character.isLetter( a[ i])&&!Character. isDigit( a[ i]))
                    return true ;
               if (!Character.isLetter( a[ i])&&!Character. isDigit( a[ i])) {
                    i++;
                    continue ;
              } else if (!Character.isLetter( a[ j])&&!Character. isDigit( a[ j])) {
                    j--;
                    continue ;
              }
               flag = true ;
               if (a [i ] == a [j ] || Math.abs(( a[ i] - a[ j])) == d ) {//若是相等，忽略大小写
                    i++;
                    j--;
              } else
                    return false ;

          }
           return true ;
     }
}