125 Valid Palindrome

125. Valid Palindrome

1. 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"isnota 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.

2. Implementation

(1) Two Pointers

class Solution {
    public boolean isPalindrome(String s) {
        if (s == null || s.length() == 0) {
            return true;
        }

        int start = 0, end = s.length() - 1;

        while (start < end) {
            if (!Character.isLetterOrDigit(s.charAt(start))) {
                ++start;
            }
            else if (!Character.isLetterOrDigit(s.charAt(end))) {
                --end;
            } 
            else if (Character.toLowerCase(s.charAt(start)) != Character.toLowerCase(s.charAt(end))) {
                return false;
            }
            else {
                ++start;
                --end;
            }
        }
        return true;
    }
}

3. Time & Space Complexity

Two Pointers: 时间复杂度O(n), 空间复杂度O(1)