convert in-order and pre-order tree presentation to post-order

post_order.py

#test cases
in_order_1 = "ABCDEFG"
pre_order_1 = "DBACEGF"
post_order_1 = "ACBFGED"

in_order_2 = "CBAD"
pre_order_2 = "BCAD"
post_order_2 = "CDAB"

def convert(in_order, pre_order):
    #get root node
    root = pre_order[0]

    #find corresponding in-order representation for left and right subtree
    in_left = in_order[:in_order.find(root)]
    in_right = in_order[in_order.find(root) + 1:]

    #recursively invoke
    left =  convert(in_left, pre_order[1:len(in_left) + 1]) if in_left else ""
    right = convert(in_right, pre_order[len(in_left) + 1:]) if in_right else ""

    #return post order
    return left + right + root
        
assert convert(in_order_1, pre_order_1) == post_order_1
assert convert(in_order_2, pre_order_2) == post_order_2
assert convert("ADEFGHMZ", "GDAFEMHZ") == "AEFDHZMG"
print "OK"