def height(root):
    nodes = {0 : [root]}
    h = 0
    while nodes.get(h):
        nodes[h+1] = []
        for node in nodes[h]:
            if node.left:
                nodes[h+1].append(node.left)
            if node.right:
                nodes[h+1].append(node.right)
        h += 1
    return max(nodes.keys()) - 1