def isBalanced(root):
    if root == None:
        return -1

    lh = isBalanced(root.left)
    rh = isBalanced(root.right)

    if lh == -2 or rh == -2:
        return -2

    if abs(lh - rh) > 1:
        return -2

    return max(lh, rh) + 1

def isBalanced(root):
    nodes = [root]
    results = []
    while nodes:
        node = nodes.pop()
        if node is None:
            results.append(-1)
        else if node == 0: # 0 is a flag we use for when we are ready to combine results
            lh = results.pop()
            rh = results.pop()
            if abs(lh - rh) > 1:
                return -2  # we could have continued with the stack; this is just a shortcut
            else:
                results.append(max(lh, rh) + 1)
        else:
            nodes.push(0)
            nodes.push(node.left)
            nodes.push(node.right)
    return results, # results will have only one value