<p>A phrase is a <strong>palindrome</strong> if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.</p>

<p>Given a string <code>s</code>, return <code>true</code><em> if it is a <strong>palindrome</strong>, or </em><code>false</code><em> otherwise</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;A man, a plan, a canal: Panama&quot;
<strong>Output:</strong> true
<strong>Explanation:</strong> &quot;amanaplanacanalpanama&quot; is a palindrome.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;race a car&quot;
<strong>Output:</strong> false
<strong>Explanation:</strong> &quot;raceacar&quot; is not a palindrome.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot; &quot;
<strong>Output:</strong> true
<strong>Explanation:</strong> s is an empty string &quot;&quot; after removing non-alphanumeric characters.
Since an empty string reads the same forward and backward, it is a palindrome.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 2 * 10<sup>5</sup></code></li>
	<li><code>s</code> consists only of printable ASCII characters.</li>
</ul>

class Solution:

    @staticmethod
    def check(s):
        return s == ' ' or not s or not (s.isalpha() or s.isdigit())

    def isPalindrome(self, s: str) -> bool:
        q = deque(list(s))

        if not q:
            return True

        l_cand, r_cand = None, None

        while True:
            try:
                if not l_cand:
                    l_cand = str(q.popleft())
                    if self.check(l_cand):
                        l_cand = None
                        continue

                if not r_cand:
                    r_cand = str(q.pop())
                    if self.check(r_cand):
                        r_cand = None
                        continue

                if l_cand and r_cand:
                    if l_cand.lower() == r_cand.lower():
                        l_cand, r_cand = None, None
                        continue

                    return False

            except IndexError:
                break

        return True
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