Merge pull request #265 from LenzyM/patch-1

Update 0707.设计链表.md

problems/0707.设计链表.md

@@ -158,6 +158,14 @@ private:
 
 Java：
 ```Java
+class ListNode {
+int val;
+ListNode next;
+ListNode(){}
+ListNode(int val) {
+this.val=val;
+}
+}
 class MyLinkedList {
     //size存储链表元素的个数
     int size;