Update LinkedList prepend pseudocode and append test (#188)

* Add LinkedList test

* Add pseudocode for LinkedList prepend

Author: hanhdt

src/data-structures/linked-list/README.md

@@ -37,7 +37,20 @@ Add(value)
   end if
 end Add
 ```
-    
+
+```text
+Prepend(value)
+ Pre: value is the value to add to the list
+ Post: value has been placed at the head of the list
+ n ← node(value)
+ n.next ← head
+ head ← n
+ if tail = ø
+   tail ← n
+ end
+end Prepend
+```
+
 ### Search
 
 ```text

src/data-structures/linked-list/__test__/LinkedList.test.js

@@ -16,6 +16,7 @@ describe('LinkedList', () => {
     linkedList.append(2);
 
     expect(linkedList.toString()).toBe('1,2');
+    expect(linkedList.tail.next).toBeNull();
   });
 
   it('should prepend node to linked list', () => {