def reverseShuffleMerge(s):
    """
    Straightforward implementation using only 2 dictionaries:
        - c_reversed - number of symbols left in a reversed part
        - c_shuffled - number of symbols left in a shuffled part
    and fixed the 'result' array to exclude the 'pop' operation.
    """
    c_reversed = {k: v // 2 for k, v in Counter(s).items()}
    c_shuffled = c_reversed.copy()
    result, rlen = [' '] * (len(s) // 2), 0

    for char in reversed(s):
        if c_reversed[char]:
            while rlen and result[rlen - 1] > char and c_shuffled[result[rlen - 1]]:
                rlen -= 1
                c_reversed[result[rlen]] += 1
                c_shuffled[result[rlen]] -= 1
            
            c_reversed[char] -= 1
            result[rlen] = char
            rlen += 1
        else:
            c_shuffled[char] -= 1

    return ''.join(result)