How to determine the time costs when inserting an element into the middle of an array implemented via ArrayList?

Inserting an element into the middle of an ArrayList requires shifting all elements after the insertion position one step to the right. This means that the time cost is proportional to the number of such elements.

If we denote the size of the list as N and the insertion position as k, then in the worst case (inserting at the beginning), almost all N elements need to be shifted, resulting in a time complexity of O(N). Inserting at the end is O(1), as no shift is needed.

Thus, inserting into the middle of an array via ArrayList has a time complexity of approximately O(N - k), where k is the insertion index.