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.

題意：檢測字符串是否爲迴文，只考慮數字和字母，空字符串視爲迴文

思路：從兩邊往中間掃描，發現非字母和數字則跳過，發現不相同直接返回false，循環結束則返回true

實現：

public class Solution {
      public boolean isPalindrome(String s ) {
           char [] a = s .toCharArray();
           int d = 'a' - 'A' ;//大寫字母與小寫字母的差值
           boolean flag = false; //檢測是否出現過字母數字，若是沒有出現且循環指向同一位置，則表示字符串空，則返回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，忽略大小寫
                    i++;
                    j--;
              } else
                    return false ;

          }
           return true ;
     }
}