LeetCode 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：

• 将所有字母变成小写
• 保留所有字母以及数字
• 判断是否是回文字符串

public class Solution {
    public boolean isPalindrome(String s) {
       if (s.length() == 0) {
			return true;
		}
		String lower = s.toLowerCase();
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < lower.length(); i++) {
			char temp = lower.charAt(i);
			if ((temp <= 'z' && temp >= 'a') || (temp <= '9' && temp >= '0') ) {
				sb.append(temp);
			}
		}
		if (sb.length() == 0) {
			return true;
		}
		int i = 0, j = sb.length() - 1 ;
		while (i < j) {
			if (sb.charAt(i) != sb.charAt(j)) {
				return false;
			}
			i++;
			j--;
		}
		return true;
	}
}