update 109

Author: fishercoder1534

Diff for: src/main/java/com/fishercoder/solutions/firstthousand/_109.java

@@ -4,9 +4,7 @@
 import com.fishercoder.common.classes.TreeNode;
 
 public class _109 {
-
     public static class Solution1 {
-
         public TreeNode sortedListToBST(ListNode head) {
             return toBstRecursively(head, null);
         }
@@ -15,16 +13,16 @@ public TreeNode toBstRecursively(ListNode start, ListNode end) {
             if (start == end) {
                 return null;
             } else {
-                ListNode mid = start;
+                ListNode slow = start;
                 ListNode fast = start;
-                while (fast != end && fast.next != end) {
-                    mid = mid.next;
+                while (fast != end && fast.next != end) {//here is the key: we check if fast != end, not fast != null
+                    slow = slow.next;
                     fast = fast.next.next;
                 }
 
-                TreeNode root = new TreeNode(mid.val);
-                root.left = toBstRecursively(start, mid);
-                root.right = toBstRecursively(mid.next, end);
+                TreeNode root = new TreeNode(slow.val);
+                root.left = toBstRecursively(start, slow);
+                root.right = toBstRecursively(slow.next, end);
                 return root;
             }
         }

Diff for: src/test/java/com/fishercoder/firstthousand/_109Test.java

@@ -5,9 +5,8 @@
 import com.fishercoder.common.utils.LinkedListUtils;
 import com.fishercoder.common.utils.TreeUtils;
 import com.fishercoder.solutions.firstthousand._109;
-import org.junit.Before;
-import org.junit.BeforeClass;
-import org.junit.Test;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
 
 import java.util.Arrays;
 
@@ -16,15 +15,11 @@ public class _109Test {
     private static ListNode head;
     private static TreeNode expected;
 
-    @BeforeClass
-    public static void setup() {
+    @BeforeEach
+    public void setup() {
         solution1 = new _109.Solution1();
     }
 
-    @Before
-    public void setUp() throws Exception {
-    }
-
     @Test
     public void test1() {
         head = LinkedListUtils.contructLinkedList(new int[]{1, 2, 3, 4, 5});