[LeetCode]Valid Palindrome

Question
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.

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本题难度Easy。

双指针法

【复杂度】
时间 O(N) 空间 O(1)

【思路】
字串中有字母、数字和其他字符，我们利用两个指针从两端向中间靠拢，碰到字母或数字就停下然后对比两端字符是否相等，否则跳过。这里要先对字串中的大小写字母进行转换。

【代码】

public class Solution {
    public boolean isPalindrome(String s) {
        //require
        if(s==null)return true;
        s=s.toLowerCase();//大小写转换
        int l=0,r=s.length()-1;
        //invariant
        while(l<r){
            if(!Character.isLetterOrDigit(s.charAt(l))){
                l++;
                continue;
            }
            if(!Character.isLetterOrDigit(s.charAt(r))){
                r--;
                continue;
            }
            if(s.charAt(l)!=s.charAt(r))
                return false;
            l++;r--;
        }
        //ensure
        return true;
    }
}