<p>Given an array of digits <code>digits</code>, return <em>the largest multiple of <strong>three</strong> that can be formed by concatenating some of the given digits in <strong>any order</strong></em>. If there is no answer return an empty string.</p>

<p>Since the answer may not fit in an integer data type, return the answer as a string. Note that the returning answer must not contain unnecessary leading zeros.</p>

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

<pre>
<strong>Input:</strong> digits = [8,1,9]
<strong>Output:</strong> &quot;981&quot;
</pre>

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

<pre>
<strong>Input:</strong> digits = [8,6,7,1,0]
<strong>Output:</strong> &quot;8760&quot;
</pre>

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

<pre>
<strong>Input:</strong> digits = [1]
<strong>Output:</strong> &quot;&quot;
</pre>

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

<ul>
	<li><code>1 &lt;= digits.length &lt;= 10<sup>4</sup></code></li>
	<li><code>0 &lt;= digits[i] &lt;= 9</code></li>
</ul>

class Solution:
    def largestMultipleOfThree(self, A):
        total = sum(A)
        count = collections.Counter(A)
        A.sort(reverse=1)

        def f(i):
            if count[i]:
                A.remove(i)
                count[i] -= 1
            if not A: return ''
            if not any(A): return '0'
            if sum(A) % 3 == 0: return ''.join(map(str, A))

        if total % 3 == 0:
            return f(-1)
        if total % 3 == 1 and count[1] + count[4] + count[7]:
            return f(1) or f(4) or f(7)
        if total % 3 == 2 and count[2] + count[5] + count[8]:
            return f(2) or f(5) or f(8)
        if total % 3 == 2:
            return f(1) or f(1) or f(4) or f(4) or f(7) or f(7)
        return f(2) or f(2) or f(5) or f(5) or f(8) or f(8)
138