Leetcode Valid Palindrome-优快云博客

本文介绍了一种算法，用于判断一个字符串是否为回文，只考虑字母数字字符且忽略大小写。通过双指针方法实现，从两端向中间比较字符。

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：

使用两个指针，一个从左到右，一个从右到左，当左指针值大于右指针时，说明String 遍历结束。注意：alphanumeric 包括 a-zA-Z0-9，一定要看清题意。

因为忽略大小写，所以就统一为小写。

同时面试时要问清楚当String is empty时，是否是valid palindrome.

Java code:

 public boolean isPalindrome(String s) {
        if(s.length() == 0){
            return true;
        }
        int p1 = 0, p2 = s.length()-1;  //p1: left to right p2: right to left
        s= s.toLowerCase();
        while(p1 <= p2){
            char left = s.charAt(p1);
            char right = s.charAt(p2);
            if(!((left >='a' && left <= 'z') || (left >= '0' && left <= '9'))){
                p1++;
                continue;
            }
            if(!((right >='a' && right <= 'z') || (right >= '0' && right <= '9'))){
                p2--;
                continue;
            }
            if(left != right){
                return false;
            }else{
                p1++;
                p2--;
            }
        }
        return true;
    }