Transforming 1level coded product tree to few level product tree

How about just a formula?

Row\Col	L	M	N	O	P	Q
1	Products	Parts	Level2	Level3
2	A	111	1818	444		N2 and across and down: =IFERROR(VLOOKUP(M2, $L$2:$M$13, 2, 0), "")
3	A	444
4	A	555
5	A	666
6	B	222
7	B	111	1818	444
8	C	333	1818	444
9	C	222
10	C	333	1818	444
11	111	1818	444
12	333	1818	444
13	1818	444

I converted all the products and parts to text first.

Copy the formula down and right until it returns a column of blanks, as shown in col P.