Adding basic Big O for ArrayList methods (#20859)

* Adding basic Big O for ArrayList methods

I added average O(n) for basic methods in arraylist. This is important for every programmer to know.

* minor edits

* moved a few lines

guide/english/java/arraylist/index.md

@@ -79,10 +79,27 @@ Since ArrayList implements *List*, an ArrayList can be created using the followi
    ```java
   Collections.reverseOrder());
  ```
-  
-  An ArrayList allows us to randomly access elements. ArrayList is similar to *Vector* in a lot of ways. But it is faster than Vectors. The main thing to note is that - Vectors are faster than arrays but ArrayLists are not. 
+ 
+   An ArrayList allows us to randomly access elements. ArrayList is similar to *Vector* in a lot of ways. But it is faster than Vectors. The main thing to note is that - Vectors are faster than arrays but ArrayLists are not. 
   
   So when it comes down to choosing between the two - if speed is critical then Vectors should be considered, otherwise ArrayLists are better when it comes to storing large number of elements and accessing them efficiently.
+ 
+ ## Basic Big O for ArrayList Methods:
+ 
+`.get(int index)`  
+- O(1). This will always be constant time. 
+    
+`.add(E ele)`  
+- O(1). We are only adding an element to the END of the list.
+
+`.add(int ind, E ele)`  
+- O(n). Because of the way an ArrayList is implemented, we must do shifting which requires O(n) time on average.
+
+`.remove(int ind)`
+- O(n). For the same reason as above. The elements must be shifted after removal.
+     
+ It is important to understand the Big O for methods of data structures. This way, you can choose the most efficient data structure for your program.
+
 
 ## More Information
 - [ArrayList Documentation](https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html)