LeetCode 125. Valid Palindrome

原题链接在这里：https://leetcode.com/problems/valid-palindrome/

题目：

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:

Input: "A man, a plan, a canal: Panama"
Output: true

Example 2:

Input: "race a car"
Output: false

题解：

注意在用Character.isLetterOrDigit(s.charAt(i)) 前需检验 i 是否index out of bound 了.

Note: if first check i < j.

Time Complexity: O(n), n = s.length().

Space: O(1).

AC Java:

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

 跟上Valid Palindrome II.