input: Write a function to implement a TicTacToe class in Python. The class should initialize a 3x3 board, display the board, and check if a player has won the game. The board should be initially filled with underscores ('_') and the first turn should be 'x'. Players take turns to place their mark ('x' or 'o'). The function should return True if the current player has won, otherwise False. If the board is full and no player has won, it should return None.
  class TicTacToe:
      def __init__(self):
          self.board = [['_' for _ in range(3)] for _ in range(3)]
          self.turn = 'x'
  
      def display(self):
          for row in self.board:
              print(' '.join(row))
  
      def check_win(self):
          board = self.board
          # check if the player with the current turn has a row of three
          for row in self.board:
              if row.count(self.turn) == 3:
                  return True
          # check if the player with the current turn has a column of three
          for col in range(3):
              if board[0][col] == board[1][col] and board[1][col] == board[2][col] and board[2][col] == self.turn:
                  return True
          # check for the two diagonals
          if board[0][0] == board[1][1] and board[1][1] == board[2][2] and board[2][2] == self.turn:
              return True
          if board[0][2] == board[1][1] and board[1][1] == board[2][0] and board[2][0] == self.turn:
              return True
          return False
  
  assert TicTacToe().check_win() == False
  assert TicTacToe().turn == 'x'
  assert TicTacToe().board == [['_', '_', '_'], ['_', '_', '_'], ['_', '_', '_']]
output: class TicTacToe:
      def __init__(self):
          self.board = [['_' for _ in range(3)] for _ in range(3)]
          self.turn = 'x'
  
      def display(self):
          for row in self.board:
              print(' '.join(row))
  
      def check_win(self):
          board = self.board
          for row in self.board:
              if row.count(self.turn) == 3:
                  return True
          for col in range(3):
              if board[0][col] == board[1][col] and board[1][col] == board[2][col] and board[2][col] == self.turn:
                  return True
          if board[0][0] == board[1][1] and board[1][1] == board[2][2] and board[2][2] == self.turn:
              return True
          if board[0][2] == board[1][1] and board[1][1] == board[2][0] and board[2][0] == self.turn:
              return True
          return False