diff --git a/Week_01/id_20/LeetCode_21 b/Week_01/id_20/LeetCode_21
new file mode 100644
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+++ b/Week_01/id_20/LeetCode_21
@@ -0,0 +1,59 @@
+/**
+ * 将两个有序链表合并为一个新的有序链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。 
+ * 
+ *  示例：
+ * 
+ *  输入：1->2->4, 1->3->4
+ *  输出：1->1->2->3->4->4
+ *
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
+        if(l1 == null) {
+            return l2;
+        }
+        if(l2 == null) {
+            return l1;
+        }
+        ListNode head = null;
+        //用于扫描l1
+        ListNode p1 = l1;
+        //用于扫描l2
+        ListNode p2 = l2;
+        if(l1.val < l2.val) {
+            head = l1;
+            p1 = l1.next;
+        } else {
+            head = l2;
+            p2 = l2.next;
+        }
+        ListNode p = head;//p指向合并列表的最新节点
+        while(p1 !=null && p2 != null) {
+            //比较插入2个链表中元素的大小
+            if(p1.val < p2.val) {
+                //p1插入p后面
+                p.next = p1;
+                p1 = p1.next;
+            } else {
+                //p2插入p后面
+                p.next = p2;
+                p2 = p2.next;
+            }
+            p = p.next;
+        } 
+
+        if(p1 != null) {
+            p.next = p1;
+        } else if(p2 != null){
+            p.next = p2;
+        }
+        return head;
+              
+    }
+}