add java solution for leetcode 21

Author: Zhanganjin0520

README.md

@@ -391,7 +391,7 @@ LeetCode
 |24|[Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs/)| [js](./algorithms/swapNodesInPairs/swapNodesInPairs.js) |Medium|
 |23|[Merge k Sorted Lists](https://leetcode.com/problems/merge-k-sorted-lists/)| |Hard|
 |22|[Generate Parentheses](https://leetcode.com/problems/generate-parentheses/)| [js](./algorithms/generateParentheses/generateParentheses.js) |Medium|
-|21|[Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)| |Easy|
+|21|[Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)| [java](./algorithm/mergeTwoSortedLists/Solution.java) |Easy|
 |20|[Valid Parentheses](https://leetcode.com/problems/valid-parentheses/)| |Easy|
 |19|[Remove Nth Node From End of List](https://leetcode.com/problems/remove-nth-node-from-end-of-list/)| [js](./algorithms/removeNthNodeFromEndOfList/removeNthNodeFromEndOfList.js) |Easy|
 |18|[4Sum](https://leetcode.com/problems/4sum/)| |Medium|

algorithms/mergeTwoSortedLists/Solution.java

@@ -0,0 +1,15 @@
+public class Solution {
+    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
+        if (null == l1) {
+            return l2;
+        } else if (null == l2) {
+            return l1;
+        } else if (l1.val < l2.val) {
+            l1.next = mergeTwoLists(l1.next, l2);
+            return l1;
+        } else {
+            l2.next = mergeTwoLists(l1, l2.next);
+            return l2;
+        }
+    }
+}