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// @Title: 两数相加 (Add Two Numbers)
// @Author: 15816537946@163.com
// @Date: 2020-08-04 09:02:43
// @Runtime: 12 ms
// @Memory: 4.9 MB
/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
	ret := new(ListNode) // 新建一个哑结点
	this := ret
	var carry int // carry 用于计算进位的位数

	// 计算的边界条件
	for l1 != nil || l2 != nil || carry > 0 {
		sum := carry 

		if l1 != nil {
			sum += l1.Val
			l1 = l1.Next
		}

		if l2 != nil {
			sum += l2.Val
			l2 = l2.Next
		}

		carry = sum / 10
		this.Next = &ListNode{Val: sum % 10}
		this = this.Next
	}

	return ret.Next
}