[ 问题: ]

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
直译：给你一个字符串， 判定它是否是回文（只统计字母、数字，其他字符请忽略）。

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.

[ 解法1: ]

先把有效的字母、数字准备好，然后遍历目标字符串，有效的字符放入buffer。

再比较buffer中的字符串和反转后的字符串，如果相同那就是回文，否则不是回文。

点评：这个解法可以被系统accept，但调用了太多api，性能一般。

public class Solution {

	public boolean isPalindrome(String s) {
		StringBuilder buffer = new StringBuilder();
		String tempStr = "abcdefghijklmnopqrstuvwxyz0123456789"; 
		char[] cArr = s.toCharArray();
		for (int i = 0; i < cArr.length; i++) { 
			if (tempStr.contains(String.valueOf(cArr[i]).toLowerCase())) {
				buffer.append(String.valueOf(cArr[i]).toLowerCase());
			}
		}
		String currentStr = buffer.toString();
		String reverseStr = buffer.reverse().toString();
		if (currentStr.equals(reverseStr)) {
			return true;
		}
		return false;
	}
	
}

【LeetCode】- Valid Palindrome（正确的回文）