Update solution 002

Author: yanglbme

README.md

@@ -27,6 +27,7 @@ Complete solutions to Leetcode problems, updated daily.
 
 | # | Title | Tags |
 |---|---|---|
+| 002 | [Add Two Numbers](https://github.com/yanglbme/leetcode/tree/master/solution/002.Add%20Two%20Numbers) | `Linked List`, `Math` |
 | 019 | [Remove Nth Node From End of List](https://github.com/yanglbme/leetcode/tree/master/solution/019.Remove%20Nth%20Node%20From%20End%20of%20List) | `Linked List`, `Two Pointers` |
 | 024 | [Swap Nodes in Pairs](https://github.com/yanglbme/leetcode/tree/master/solution/024.Swap%20Nodes%20in%20Pairs) | `Linked List` |
 | 082 | [Remove Duplicates from Sorted List II](https://github.com/yanglbme/leetcode/tree/master/solution/082.Remove%20Duplicates%20from%20Sorted%20List%20II) | `Linked List` |

solution/002.Add Two Numbers/README.md

@@ -0,0 +1,101 @@
+## 两数相加
+### 题目描述
+
+给定两个非空链表来表示两个非负整数。位数按照逆序方式存储，它们的每个节点只存储单个数字。将两数相加返回一个新的链表。
+
+你可以假设除了数字 0 之外，这两个数字都不会以零开头。
+
+示例：
+```
+输入：(2 -> 4 -> 3) + (5 -> 6 -> 4)
+输出：7 -> 0 -> 8
+原因：342 + 465 = 807
+```
+
+### 解法
+同时遍历两个链表，对应值相加(还有 quotient)求余数得到值并赋给新创建的结点。而商则用quotient存储，供下次相加。
+
+初始版本：
+
+```java
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
+        ListNode res = new ListNode(-1);
+        ListNode cur = res;
+        int quotient = 0;
+        int t = 0;
+        while (l1 != null && l2 != null) {
+            t = l1.val + l2.val + quotient;
+            quotient = t / 10;
+            ListNode node = new ListNode(t % 10);
+            cur.next = node;
+            l1 = l1.next;
+            l2 = l2.next;
+            cur = node;
+        }
+        
+        while (l1 != null) {
+            t = l1.val + quotient;
+            quotient = t / 10;
+            ListNode node = new ListNode(t % 10);
+            cur.next = node;
+            l1 = l1.next;
+            cur = node;
+        }
+        
+        while (l2 != null) {
+            t = l2.val + quotient;
+            quotient = t / 10;
+            ListNode node = new ListNode(t % 10);
+            cur.next = node;
+            l2 = l2.next;
+            cur = node;
+        }
+        
+        if (quotient != 0) {
+            cur.next = new ListNode(quotient);
+            cur = cur.next;
+        }
+        
+        return res.next;
+        
+    }
+}
+```
+
+简化版本：
+```java
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
+        ListNode res = new ListNode(-1);
+        ListNode cur = res;
+        int quotient = 0;
+        while (l1 != null || l2 != null || quotient != 0) {
+            int t = (l1 == null ? 0 : l1.val) + (l2 == null ? 0 : l2.val) + quotient;
+            quotient = t / 10;
+            ListNode node = new ListNode(t % 10);
+            cur.next = node;
+            cur = node;
+            l1 = (l1 == null) ? l1 : l1.next;
+            l2 = (l2 == null) ? l2 : l2.next;
+        }
+        return res.next;
+    }
+}
+```

solution/002.Add Two Numbers/Solution.java

@@ -0,0 +1,17 @@
+class Solution {
+    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
+        ListNode res = new ListNode(-1);
+        ListNode cur = res;
+        int quotient = 0;
+        while (l1 != null || l2 != null || quotient != 0) {
+            int t = (l1 == null ? 0 : l1.val) + (l2 == null ? 0 : l2.val) + quotient;
+            quotient = t / 10;
+            ListNode node = new ListNode(t % 10);
+            cur.next = node;
+            cur = node;
+            l1 = (l1 == null) ? l1 : l1.next;
+            l2 = (l2 == null) ? l2 : l2.next;
+        }
+        return res.next;
+    }
+}