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website/content/ChapterFour/0002.Add-Two-Numbers.md

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+# [2. Add Two Numbers](https://leetcode.com/problems/add-two-numbers/)
+
+## 题目
+
+You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.
+
+You may assume the two numbers do not contain any leading zero, except the number 0 itself.
+
+**Example**:
+
+```
+
+Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
+Output: 7 -> 0 -> 8
+Explanation: 342 + 465 = 807.
+```
+
+
+
+## 题目大意
+
+2 个逆序的链表，要求从低位开始相加，得出结果也逆序输出，返回值是逆序结果链表的头结点。
+
+## 解题思路
+
+需要注意的是各种进位问题。
+
+极端情况，例如
+
+```
+
+Input: (9 -> 9 -> 9 -> 9 -> 9) + (1 -> )
+Output: 0 -> 0 -> 0 -> 0 -> 0 -> 1
+
+
+```
+
+为了处理方法统一，可以先建立一个虚拟头结点，这个虚拟头结点的 Next 指向真正的 head，这样 head 不需要单独处理，直接 while 循环即可。另外判断循环终止的条件不用是 p.Next ！= nil，这样最后一位还需要额外计算，循环终止条件应该是 p != nil。
+
+
+## 代码
+
+```go
+
+package leetcode
+
+/**
+ * Definition for singly-linked list.
+ * type ListNode struct {
+ *     Val int
+ *     Next *ListNode
+ * }
+ */
+func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
+	if l1 == nil || l2 == nil {
+		return nil
+	}
+	head := &ListNode{Val: 0, Next: nil}
+	current := head
+	carry := 0
+	for l1 != nil || l2 != nil {
+		var x, y int
+		if l1 == nil {
+			x = 0
+		} else {
+			x = l1.Val
+		}
+		if l2 == nil {
+			y = 0
+		} else {
+			y = l2.Val
+		}
+		current.Next = &ListNode{Val: (x + y + carry) % 10, Next: nil}
+		current = current.Next
+		carry = (x + y + carry) / 10
+		if l1 != nil {
+			l1 = l1.Next
+		}
+		if l2 != nil {
+			l2 = l2.Next
+		}
+	}
+	if carry > 0 {
+		current.Next = &ListNode{Val: carry % 10, Next: nil}
+	}
+	return head.Next
+}
+
+```