2. Add Two Numbers.go

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
}