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.
参考：
http://www.cnblogs.com/jdflyfly/p/3819162.html
Cracking the Coding Interview Q2.5,布布扣,bubuko.com
Cracking the Coding Interview Q2.5
原文：http://www.cnblogs.com/jdflyfly/p/3830693.html