Suggest an efficient algorithm for removing multiple adjacent elements from the middle of a list implemented using ArrayList.

The most efficient way to remove multiple adjacent elements from the middle of an ArrayList is to use the removeRange method.

import java.util.ArrayList;
import java.util.List;

public class ArrayListRemoveExample {

    public static void main(String[] args) {
        List<Integer> list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            list.add(i);
        }
        System.out.println("Original list: " + list);

        int startIndex = 3; // Starting index (inclusive)
        int endIndex = 6;   // Ending index (exclusive)

        ((ArrayList<Integer>) list).removeRange(startIndex, endIndex); // Remove elements from startIndex to endIndex-1 inclusive

        System.out.println("List after removing elements: " + list);
    }
}

The removeRange method is a protected method of the ArrayList class, so it must be explicitly cast to ArrayList to use it. This method is implemented most efficiently in terms of shifting elements.

The complexity of removing k elements from the middle of an ArrayList is O(n-i), where n is the size of the list, and i is the starting index of removal. removeRange shifts elements in a single pass. Repeatedly using remove(int index) for each element to be removed would result in a complexity of O(k * (n-i)), which is less efficient.