def getHash(expression):
    """
    Create a hash of all characters for an expression, every character has
    a sign prefixed, so we decrease the count of character if its a '-' else
    we keep increasing. We remove the characters if their count is zero
    """
    h = {}
    for i in range(0, len(expression), 2):
        sign = expression[i]
        char = expression[i+1]
        if char not in h:
            h[char] = 0
        if sign == "+":
            h[char] += 1
        else:
            h[char] -= 1
        if h[char] == 0:
            del h[char]
    return h