Check amplitude of a tree in Python language (recursive)

amplitudes.py

def solution(T):
    amplitudes = []
    if (T[1] is None and T[2] is None):
        return 0
    if (T[1] is not None):
        walk(T[1], T[0], T[0], amplitudes)
    if (T[2] is not None):
        walk(T[2], T[0], T[0], amplitudes)
    return max(amplitudes)


def walk(node, m, x, amplitudes):
    """ iterate tree in pre-order depth-first search order """
    m = min(node[0], m)
    x = max(node[0], x)
    if (node[1] is not None):
        walk(node[1], m, x, amplitudes)
    if (node[2] is not None):
        walk(node[2], m, x, amplitudes)
    if (node[1] is None and node[2] is None):
        amplitudes.append(abs(x-m))


print solution( (1, None, None) )
t = (5, (8, (12, None, None), (2, None, None)),
      (9, (7, (1, None, None), None), (4, (3, None, None), None)))
print solution(t)