Update Solution.java

src/solution/ListNode.java

@@ -0,0 +1,20 @@
+package solution;
+
+public class ListNode {
+    int val;
+    ListNode next;
+
+    ListNode() {
+    }
+
+    ListNode(int val) {
+        this.val = val;
+    }
+
+    ListNode(int val, ListNode next) {
+        this.val = val;
+        this.next = next;
+    }
+}
+
+

src/solution/Solution.java

@@ -204,4 +204,26 @@ public class Solution {
         }
         return true;
     }
+
+
+    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
+        ListNode dummy = new ListNode(0);
+        ListNode current  = dummy;
+        while (list1 != null && list2 != null) {
+            if (list1.val < list2.val) {
+                current.next = list1;
+                list1 = list1.next;
+            } else {
+                current.next = list2;
+                list2 = list2.next;
+            }
+            current = current.next;
+        }
+        if (list1 != null){
+            current.next = list1;
+        } else if (list2 != null){
+            current.next = list2;
+        }
+        return dummy.next;
+    }
 }