注意的问题：
1.进位
2.两个list的长度不一定相等
func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
result := &ListNode{0, nil}
cursor := result
leftBit, rightBit, carryBit := 0, 0, 0
for l1 != nil || l2 != nil || carryBit > 0 {
if l1 != nil {
leftBit = l1.Val
l1 = l1.Next
} else {
leftBit = 0
}
if l2 != nil {
rightBit = l2.Val
l2 = l2.Next
} else {
rightBit = 0
}
//考虑有进位的情况
cursor.Val = (leftBit + rightBit + carryBit) % 10
carryBit = (leftBit + rightBit + carryBit) / 10
if l1 != nil || l2 != nil || carryBit > 0 {
cursor.Next = &ListNode{0, nil}
cursor = cursor.Next
}
}
return result
}
