def _longest(ts):
if ts == []: return 0, 0
ldepth, lpath = _longest(ts[0])
rdepth, rpath = _longest(ts[2])
depth = max(ldepth, rdepth) + 1
cpath = max(lpath, rpath)
return depth, max(ldepth+rdepth, cpath)
def longest(ts):
return _longest(ts)[1]