/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
    // 记录进位
    carry := 0
    var dummyHead ListNode
    // 指向三个链表的指针
    p1, p2, p3 := l1, l2, &dummyHead
    for p1 != nil || p2 != nil {
        v1, v2 := 0, 0
        if p1 != nil {
            v1 = p1.Val
            p1 = p1.Next
        }
        if p2 != nil {
            v2 = p2.Val
            p2 = p2.Next
        }
        sum := v1 + v2 + carry
        carry = sum / 10
        p3.Next = &ListNode{Val : sum % 10}
        p3 = p3.Next
    }
    // 注意检查最后的进位!
    if carry == 1 {
        p3.Next = &ListNode{Val : 1}
    }
    return dummyHead.Next
}