[LeetCode] Merge Two Sorted Lists

Author: codehumane

src/main/java/linkedlist/MergeTwoSortedLists.java

@@ -0,0 +1,40 @@
+package linkedlist;
+
+/**
+ * Merge two sorted linked lists and return it as a new sorted list.
+ * The new list should be made by splicing together the nodes of the first two lists.
+ * <p>
+ * Example:
+ * <p>
+ * Input: 1->2->4, 1->3->4
+ * Output: 1->1->2->3->4->4
+ */
+public class MergeTwoSortedLists {
+
+    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
+        if (l1 == null) return l2;
+        if (l2 == null) return l1;
+
+        ListNode head = new ListNode(-1);
+        ListNode c = head;
+        ListNode c1 = l1;
+        ListNode c2 = l2;
+
+        while (c1 != null || c2 != null) {
+
+            if (c1 == null || (c2 != null && c1.val > c2.val)) {
+                c.next = c2;
+                c = c.next;
+                c2 = c2.next;
+            } else {
+                c.next = c1;
+                c = c.next;
+                c1 = c1.next;
+            }
+
+        }
+
+        return head.next;
+    }
+
+}

src/test/java/linkedlist/MergeTwoSortedListsTest.java

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+package linkedlist;
+
+import lombok.val;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class MergeTwoSortedListsTest {
+
+    @Test
+    public void mergeTwoListsExample1() {
+
+        // given: 1->2->4, 1->3->4
+        val nodeA1 = new ListNode(1);
+        val nodeA2 = new ListNode(2);
+        val nodeA3 = new ListNode(4);
+        val nodeB1 = new ListNode(1);
+        val nodeB2 = new ListNode(3);
+        val nodeB3 = new ListNode(4);
+        nodeA1.next = nodeA2;
+        nodeA2.next = nodeA3;
+        nodeB1.next = nodeB2;
+        nodeB2.next = nodeB3;
+
+        // when
+        val merged = new MergeTwoSortedLists().mergeTwoLists(nodeA1, nodeB1);
+
+        // then: 1->1->2->3->4->4
+        assertEquals(nodeA1.val, merged.val);
+        assertEquals(nodeB1.val, merged.next.val);
+        assertEquals(nodeA2.val, merged.next.next.val);
+        assertEquals(nodeB2.val, merged.next.next.next.val);
+        assertEquals(nodeA3.val, merged.next.next.next.next.val);
+        assertEquals(nodeB3.val, merged.next.next.next.next.next.val);
+    }
+
+    @Test
+    public void mergeTwoListsExample2() {
+        // given: 2, 1
+        val nodeA1 = new ListNode(2);
+        val nodeB1 = new ListNode(1);
+
+        // when
+        val merged = new MergeTwoSortedLists().mergeTwoLists(nodeA1, nodeB1);
+
+        // then: 1->2
+        assertEquals(nodeB1.val, merged.val);
+        assertEquals(nodeA1.val, merged.next.val);
+    }
+
+}