def postordereval(tree):
    opers = {'+':operator.add, '-':operator.sub, 
             '*':operator.mul, '/':operator.div}
    res1 = None
    res2 = None
    if tree:
        res1 = postordereval(tree.getLeftChild())          res2 = postordereval(tree.getRightChild())         if res1 and res2:
            return opers[tree.getRootVal()](res1,res2)         else:
            return tree.getRootVal()