leetcode Add Two Numbers

题意：给定两个数字，这两个数字用链表表示，每个节点保存一位数字，表头保存着个位数。求相加的和。

思路：模拟加法的原理，该方法也可以用于大整数相加，比如说1000为的整数相加，数字无法用任何类型保存，所以用字符串保存然后模仿加法。

代码示例：/**
* 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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
ListNode *p = l1, *q = l2, *head = new ListNode(0), *r = head;
int carry = 0;
while(p != NULL || q != NULL)
{
ListNode *temp = r;
int number = 0;
if(p != NULL)
number += p->val;
if(q != NULL)
number += q->val;
number += carry;
carry = number / 10;
number = number % 10;
r = new ListNode(number);
temp->next = r;
if(p != NULL)
p = p->next;
if(q != NULL)
q = q->next;
}
if(carry)
{
ListNode *temp = r;
r = new ListNode(1);
temp->next = r;
}
return head->next;
}
};