125 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.

ASCII: [A-Z]:65-90, [a-z]:97-122

Character.isLetterOrDigit() Character.isLetter()

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

    int f = 0, e = s.length()-1;
    s = s.toUpperCase();

    while (f < e) {
        if (!Character.isLetterOrDigit(s.charAt(f))) {
            f++;
            continue;
        }
        if (!Character.isLetterOrDigit(s.charAt(e))) {
            e--;
            continue;
        }
        if (s.charAt(f) != s.charAt(e)) {
            return false;
        }
        f++;
        e--;
    }
    return true;
}

}

updated(12.18.15)

public class Solution {
    public boolean isPalindrome(String str) {
        //becasue we need to ignoring something and compare from two end, so we need two pointers
        if (str == null) {
            return true;
        }
        str = str.trim();
        if (str.length() == 0)  {
            return true;
        }
        // toLowerCase(), return string.
        str = str.toLowerCase();
        int i = 0, j = str.length() - 1;
        while (i < j) {
            // continue相当于说，必须满足我才能进行循环的其他项，否则就连续先保证满足我。
            // 审题不认真，没注意只关心字母。
            if (!Character.isLetterOrDigit(str.charAt(i))) {
                i++;
                continue;
            }
            if (!Character.isLetterOrDigit(str.charAt(j))) {
                j--;
                continue;
            }
            if (str.charAt(i) != str.charAt(j)) {
                return false;
            }
            // 看到循环，一定提醒自己写终止条件
            i++;
            j--;
        }
        return true;
    }
}