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website/content/ChapterFour/0445.Add-Two-Numbers-II.md

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+# [445. Add Two Numbers II](https://leetcode.com/problems/add-two-numbers-ii/)
+
+## 题目
+
+You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first 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.
+
+**Follow up**:
+What if you cannot modify the input lists? In other words, reversing the lists is not allowed.
+
+**Example**:
+
+```
+
+Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
+Output: 7 -> 8 -> 0 -> 7
+
+```
+
+## 题目大意
+
+这道题是第 2 题的变种题，第 2 题中的 2 个数是从个位逆序排到高位，这样相加只用从头交到尾，进位一直进位即可。这道题目中强制要求不能把链表逆序。2 个数字是从高位排到低位的，这样进位是倒着来的。
+
+## 解题思路
+
+思路也不难，加法只用把短的链表依次加到长的链表上面来就可以了，最终判断一下最高位有没有进位，有进位再往前进一位。加法的过程可以用到递归。
+
+## 代码
+
+```go
+
+package leetcode
+
+/**
+ * Definition for singly-linked list.
+ * type ListNode struct {
+ *     Val int
+ *     Next *ListNode
+ * }
+ */
+func addTwoNumbers445(l1 *ListNode, l2 *ListNode) *ListNode {
+	if l1 == nil {
+		return l2
+	}
+	if l2 == nil {
+		return l1
+	}
+	l1Length := getLength(l1)
+	l2Length := getLength(l2)
+	newHeader := &ListNode{Val: 1, Next: nil}
+	if l1Length < l2Length {
+		newHeader.Next = addNode(l2, l1, l2Length-l1Length)
+	} else {
+		newHeader.Next = addNode(l1, l2, l1Length-l2Length)
+	}
+	if newHeader.Next.Val > 9 {
+		newHeader.Next.Val = newHeader.Next.Val % 10
+		return newHeader
+	}
+	return newHeader.Next
+}
+
+func addNode(l1 *ListNode, l2 *ListNode, offset int) *ListNode {
+	if l1 == nil {
+		return nil
+	}
+	var (
+		res, node *ListNode
+	)
+	if offset == 0 {
+		res = &ListNode{Val: l1.Val + l2.Val, Next: nil}
+		node = addNode(l1.Next, l2.Next, 0)
+	} else {
+		res = &ListNode{Val: l1.Val, Next: nil}
+		node = addNode(l1.Next, l2, offset-1)
+	}
+	if node != nil && node.Val > 9 {
+		res.Val++
+		node.Val = node.Val % 10
+	}
+	res.Next = node
+	return res
+}
+
+func getLength(l *ListNode) int {
+	count := 0
+	cur := l
+	for cur != nil {
+		count++
+		cur = cur.Next
+	}
+	return count
+}
+
+```