code: def add_child(self, u, v):
          ''' add child to search tree itself.
          Arguments:
              u {int} -- father id
              v {int} --  child id
          '''
  
          if u == -1:
              self.root = v
              self.adj_list[v] = []
              return
          if v not in self.adj_list[u]:
              self.adj_list[u].append(v)
          if v not in self.adj_list:
              self.adj_list[v] = [f add_child(self, u, v):
          ''' add child to search tree itself.
          Arguments:
              u {int} -- father id
              v {int} --  child id
          '''
  
          if u == -1:
              self.root = v
              self.adj_list[v] = []
              return
          if v not in self.adj_list[u]:
              self.adj_list[u].append(v)
          if v not in self.adj_list:
              self.adj_list[v] = []
docs: add child to search tree itself.
          Arguments:
              u {int} -- father id
              v {int} --  child id