MCTS Code explanation

[Ishtdeep-TDA]

Hi,

My question is that in the search function in MCTS, what is considered to be a leaf node? Does the leaf node mean a terminal state has been reached? If so, then why do we return if s not in self.Ps, Does'nt that just mean that a new state has been reached which is not a terminal state and so the search should continue until we do find a terminal state.