2. Add Two Numbers

Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.
exaple:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)Output: 7 -> 0 -> 8

思路：
理解题目为，数字两两相加，并朝下一位进位。
情况一：其中一个列表提前加完，将没加完的补充到需要输出的列表处。
情况二：同时加完，却仍有进位，需创建一个结点存储。

代码：

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {        int up = 0;        ListNode *pNode = NULL, *pNext = NULL, *resList = NULL; //定义ListNode类型的指针        ListNode *p = l1, *q = l2;        while (p != NULL && q != NULL){            pNext = new ListNode(q->val + p->val + up);//将指针指向新的结点            up = pNext->val / 10;            pNext->val = pNext->val % 10;            if(resList == NULL){                resList = pNode = pNext;//保持头结点的不动            }            else{                pNode->next = pNext;//结点连接                pNode = pNext;             }            p = p->next;            q = q->next;        }        while (q != NULL){            //l2仍有剩余            pNext = new ListNode(q->val + up);            up = pNext->val / 10;            pNext->val = pNext->val % 10;            pNode->next = pNext;//结点连接            pNode = pNext;             q = q->next;        }        while (p != NULL){            //l1仍有剩余            pNext = new ListNode(p->val + up);            up = pNext->val / 10;            pNext->val = pNext->val % 10;            pNode->next = pNext;//结点连接            pNode = pNext;             p = p->next;        }        if (up != 0){            //都没有剩余，却仍有进位，需要再创建一个点            pNext = new ListNode(up);            pNode->next = pNext;            pNode = pNext;        }        return resList; //返回结果的头指针    }};

知识点：指针、列表的使用，指针的向下移动