leetcode2 add two numbers

i used two approached, one is with additional stack, another is reusing the linked list of the longer one to reduce the space usage.so i compare the length of them firstly.
in the official solution, they just create new nodes, and link them naturally, it is very clean. while i don't know why i create a dummy head, and another node pointing to dummy head, it makes code complicated, so confusing why i do that.
they only have two condition statement, if l1 exist, add it to summary result, if l2 exist, add to the summary result too. it is so clean. why i have there condition statement, one is both l1 and l2 exist, one is only l1 exist, and else. it's complicated and not that readable.