Description：

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.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4) Output: 7 -> 0 -> 8 Explanation: 342 + 465 = 807.

注意邊界： 1.不能將兩個加數從連結串列裡面取出來，加完將和再用連結串列表示，因為這些數可 能會大於int、float、double型的範圍； 2.返回的連結串列，尾巴是否正確指向NULL？ 3.推薦測試用例： 0+0=0 5+5=10 9999999991+9=10000000000

struct ListNode* addTwoNumbers(struct ListNode*
 
 l1, struct ListNode* l2) {
   int num,carry=0,bit;
	struct ListNode* p1=l1, *p2=l2;
	struct ListNode* l3 = (struct ListNode*)malloc(sizeof(struct ListNode));
	num = (p1->val) + (p2->val);
	if (num / 10 != 0)	carry = 1;
	bit = num % 10;
	l3->val = bit;
	l3->next = NULL;
	struct ListNode* p3 =
 
 l3;
	p1 = p1->next;
	p2 = p2->next;

	//add
	while (p1&&p2)
	{
		num = (p1->val) + (p2->val)+carry;
		carry = 0;
		if (num / 10 != 0)	carry = 1;
		bit = num % 10;

		struct ListNode* new_node = (struct ListNode*)malloc(sizeof(struct ListNode));
		new_node->val = bit;
		new_node->next = NULL;
		p3->next = new_node;
		p3 = p3->next;
		p1 = p1->next;
		p2 = p2->next;
	}

	if (p1)
	{
		while (carry != 0&&p1!=NULL)
		{
			num = (p1->val) + carry;
			carry = num / 10;
			bit = num % 10;
			struct ListNode* new_node = (struct ListNode*)malloc(sizeof(struct ListNode));
			new_node->val = bit;
			new_node->next = NULL;
			p3->next = new_node;
			p3 = p3->next;
			p1 = p1->next;
		}
		if (carry == 0 && p1 == NULL)
		{
			p3->next = NULL;
			return l3;
		}
		else if (carry == 0)
		{
			p3->next = p1;
			return l3;
		}
		else if (p1 == NULL)
		{
			struct ListNode* new_node = (struct ListNode*)malloc(sizeof(struct ListNode));
			new_node->val = 1;
			new_node->next = NULL;
			p3->next = new_node;
			return l3;
		}
	}

	else if (p2)
	{
		while (carry != 0 && p2 != NULL)
		{
			num = (p2->val) + carry;
			carry = num / 10;
			bit = num % 10;
			struct ListNode* new_node = (struct ListNode*)malloc(sizeof(struct ListNode));
			new_node->val = bit;
			new_node->next = NULL;
			p3->next = new_node;
			p3 = p3->next;
			p2 = p2->next;
		}
		if (carry == 0 && p2 == NULL)
		{
			p3->next = NULL;
			return l3;
		}
		else if (carry == 0)
		{
			p3->next = p2;
			return l3;
		}
		else if (p2 == NULL)
		{
			struct ListNode* new_node = (struct ListNode*)malloc(sizeof(struct ListNode));
			new_node->val = 1;
			new_node->next = NULL;
			p3->next = new_node;
			return l3;
		}
	}

	else if (carry == 1)
	{
		struct ListNode* new_node = (struct ListNode*)malloc(sizeof(struct ListNode));
		new_node->val = 1;
		new_node->next = NULL;
		p3->next = new_node;
	}
	return l3;
}