Java [Leetcode 125]Valid Palindrome

题目描述：

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.

解题思路：

设置两个指针，一个在字符串头部，一个在字符串尾部，分别向中间移动，遇到非字母或数字则继续向中间移动，如两个都为字母或者数字，那么则比较两者是否相同。

代码如下：

public class Solution {
public boolean isPalindrome(String s) {
int i = 0, j = s.length() - 1;
char head, tail;
if(j < 0)
return true;
while(i < j){
head = s.charAt(i);
tail = s.charAt(j);
if(!Character.isLetterOrDigit(head)){
i++;
}
if(!Character.isLetterOrDigit(tail)){
j--;
}
if(Character.isLetterOrDigit(head) && Character.isLetterOrDigit(tail)){
if(Character.toLowerCase(head) != Character.toLowerCase(tail)){
return false;
}
i++;
j--;
}
}
return true;
}
}