transition.py

 # sentence :The sentence to be parsed as a list of words.
 self.stack = ["ROOT"] # corresponds to first column in above image 
 self.buffer = (self.sentence).copy() # corresponds to second column in above image
 self.dependencies = [] # corresponds to the third column in above image
 
 # transition (str): A string that equals "S", "LA", or "RA" representing the shift,
 #                   left-arc, and right-arc transitions. You can assume the provided
 #                   transition is a legal transition.
 #                   corresponds to the last column in above image
 if transition == "S":
     self.stack.append(self.buffer.pop(0))
 elif transition == "LA":   
     self.dependencies.append((self.stack[-1],self.stack[-2]))
     self.stack.pop(-2)
 elif transition == "RA":
     self.dependencies.append((self.stack[-2],self.stack[-1]))
     self.stack.pop(-1)