《CITI》P123
2 鏈表
題目：
2.5 You have two numbers represented by a linked list, where each node contains a single digit. The digits are stored in reverse order, such that the 1 ‘s digit is at the head of the list. Write a function that adds the two numbers and returns the sum as a linked list.
FOLLOW UP
Suppose the digits are stored in forward order. Repeat the above problem.
