测试用例1500多个，考虑的情况非常多

[0]  [1]    [2]  [3,4,2]  [5,0] [2]  [1]
[0]  [9,8]  [0]  [4,6,5]  [5,0] [1]  [9]
[0]  [0,9]  [2]  [8,0,7]  [0,1] [3]  [0,1]

func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
    result := &ListNode{}
	lastNode := result
	temp := 0
	for l1 != nil || l2 != nil || temp != 0 {
		//相加处理
		var v1, v2 int
		if l1 != nil {
			v1 = l1.Val
			//移位
			l1 = l1.Next
		} else {
			v1 = 0
		}
		if l2 != nil {
			v2 = l2.Val
			//移位
			l2 = l2.Next
		} else {
			v2 = 0
		}
		sum := v1 + v2 + temp
		temp = sum / 10
		bitNum := sum % 10
 
		//将数据添加到末端节点
		lastNode.Val = bitNum
		//是否准备新节点
		if l1 != nil || l2 != nil || temp != 0 {
			lastNode.Next = &ListNode{}
		} else {
			lastNode.Next = nil
		}
		lastNode = lastNode.Next
	}
	return result
}