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：判断给定的字符串是否为回文。回文，亦称回环，是正读反读都能读通的句子，亦有将文字排列成圆圈者。

思路：通过从前后两个方向来判断是否为回文。本题中有效的字符是字母和数字，并且忽略字母的大小写。

代码：

public class Solution {    //判断是否为小写、大写、数字    public boolean isalpha(char c){        if('a' <= c && c <= 'z') return true;          if('A' <= c && c <= 'Z') return true;          if('0' <= c && c <= '9') return true;          return false;      }    //将大写转换为小写    public char tolower(char c){         if ((c>='A'&&c<='Z')){             return c = (char) (c-'A' + 'a');         }        return c;     }    public boolean isPalindrome(String s) {            int n = s.length();            int i = 0,j= n - 1;            while(i

结果：