What affects the greatest time complexity when inserting an element into an ArrayList?

The highest time complexity when inserting an element into an ArrayList occurs when the element is inserted not at the end of the list, but in the middle or at the beginning. In this case, all elements starting from the insertion position need to be shifted to the right to make space for the new element. This results in a copy operation of elements, which takes time proportional to the number of shifted elements.

Thus, the time complexity of inserting into an ArrayList:

• At the end: amortized O(1) (if the array does not need to be resized)
• In the middle or at the beginning: O(n), where n is the number of elements after the insertion position

Example:

ArrayList<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3, 4));
list.add(1, 99); // insert at position 1, elements from position 1 onward are shifted