125.Valid Palindrome

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Test cases

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"0P"
"ab"
",."
",a,"
",aa."
""
"a"
"bab"
"baab"
"babb"

Solution 1: accepted 10ms

Two pointers. 

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public boolean isPalindrome(String s) {
    s = s.toLowerCase();
    int l = 0;
    int r = s.length() - 1;
    while (l < r) {
        char lc = s.charAt(l);
        char rc = s.charAt(r);
        if (lc < 48 || (57 < lc && lc < 97) || lc > 122) {
            l++;
            continue;
        }
        if (rc < 48 || (57 < rc && rc < 97) || rc > 122) {
            r--;
            continue;
        }
        if (lc != rc) {
            System.out.println(lc + " " + rc);
            return false;
        } else {
            l++;
            r--;
        } 
    }
    if(l < r && s.charAt(l) != s.charAt(r)) // two left in the middle
        return false;
    return true;
}

Improved to 8ms using Character functions.

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public boolean isPalindrome(String s) {
    int l = 0;
    int r = s.length() - 1;
    while (l < r) {
        char lc = s.charAt(l);
        char rc = s.charAt(r);
        if (!Character.isLetterOrDigit(lc)) {
            l++;
            continue;
        }
        if (!Character.isLetterOrDigit(rc)) {
            r--;
            continue;
        }
        if (Character.toLowerCase(lc) != Character.toLowerCase(rc)) {
            return false;
        } else {
            l++;
            r--;
        } 
    }
    if(l < r && s.charAt(l) != s.charAt(r)) // two left in the middle
        return false;
    return true;
}