Minor changes in main and Junit tests implemented

Akshith121 · Akshith121 · commit c9fc75a3f06b · 2023-04-08T22:40:57.000+05:30
diff --git a/src/main/java/com/thealgorithms/datastructures/lists/SinglyLinkedList.java b/src/main/java/com/thealgorithms/datastructures/lists/SinglyLinkedList.java
@@ -122,7 +122,11 @@ public void swapNodes(int valueFirst, int valueSecond) {
      * Reverse a singly linked list from a given node till the end
      *
      */
-    Node reverseList(Node node) {
+    public Node reverseList(Node node) {
+        //When node is null, then "next" will cause an error as we are accesing node.next, which will give a NullPointer Exception
+        if(node==null){
+            return node;
+        }
         Node prev = null;
         Node curr = node;
         Node next=curr.next;
@@ -135,7 +139,7 @@ Node reverseList(Node node) {
                 next = next.next;
             }
         }
-        //prev will be pointing to the last element in the Linkedlist
+        //prev will be pointing to the last element in the Linkedlist, it will be the new head of the linkedlist
         return prev;
     }
 
diff --git a/src/test/java/com/thealgorithms/datastructures/lists/SinglyLinkedListTest.java b/src/test/java/com/thealgorithms/datastructures/lists/SinglyLinkedListTest.java
@@ -1,5 +1,6 @@
 package com.thealgorithms.datastructures.lists;
 
+
 import org.junit.jupiter.api.Test;
 
 import java.util.ArrayList;
@@ -99,4 +100,42 @@ void deleteNth() {
         list.deleteNth(6); //Index 6 has value 7
         assertFalse(list.search(7));
     }
+    //Test to check whether the method reverseList() works fine
+    @Test
+    void reverseList(){
+
+        //Creating a new LinkedList of size:4
+        //The linkedlist will be 1->2->3->4->null
+        SinglyLinkedList list = createSampleList(4);
+          
+        //Reversing the LinkedList using reverseList() method and storing the head of the reversed linkedlist in a head node
+        //The reversed linkedlist will be 4->3->2->1->null
+        Node head=list.reverseList(list.getHead());
+
+        //Recording the Nodes after reversing the LinkedList
+        Node firstNode = head;              //4
+        Node secondNode = firstNode.next;   //3
+        Node thirdNode = secondNode.next;   //2
+        Node fourthNode = thirdNode.next;   //1
+        
+        //Checking whether the LinkedList is reversed or not by comparing the original list and reversed list nodes
+        assertEquals(1,fourthNode.value);
+        assertEquals(2,thirdNode.value);
+        assertEquals(3,secondNode.value);
+        assertEquals(4,firstNode.value);
+    }
+    
+    //Test to check whether implemented reverseList() method handles NullPointer Exception for TestCase where head==null
+    @Test
+    void reverseListNullPointer(){
+        SinglyLinkedList list=new SinglyLinkedList();
+        Node first=list.getHead();
+
+        list.reverseList(first);
+
+        Node newFirst=list.getHead();
+        assertEquals(first,newFirst);
+
+    }
+
 }

Original file line number	Diff line number	Diff line change
`@@ -122,7 +122,11 @@ public void swapNodes(int valueFirst, int valueSecond) {`
`122`	`122`	`* Reverse a singly linked list from a given node till the end`
`123`	`123`	`*`
`124`	`124`	`*/`
`125`		`- Node reverseList(Node node) {`
	`125`	`+ public Node reverseList(Node node) {`
	`126`	`+ //When node is null, then "next" will cause an error as we are accesing node.next, which will give a NullPointer Exception`
	`127`	`+ if(node==null){`
	`128`	`+ return node;`
	`129`	`+ }`
`126`	`130`	`Node prev = null;`
`127`	`131`	`Node curr = node;`
`128`	`132`	`Node next=curr.next;`
`@@ -135,7 +139,7 @@ Node reverseList(Node node) {`
`135`	`139`	`next = next.next;`
`136`	`140`	`}`
`137`	`141`	`}`
`138`		`- //prev will be pointing to the last element in the Linkedlist`
	`142`	`+ //prev will be pointing to the last element in the Linkedlist, it will be the new head of the linkedlist`
`139`	`143`	`return prev;`
`140`	`144`	`}`
`141`	`145`