Having difficulty identifying the next step compiler will take after a recursive call

Question

I am having difficulty understanding this code.

class Solution {
    public List<List<Integer>> subsets(int[] nums) {
     List<List<Integer>> subsets = new ArrayList<>();
     generateSubsets(0, nums, new ArrayList<Integer>(), subsets);
     return subsets;
    }
    public void generateSubsets(int index, int[] nums, List<Integer> current, List<List<Integer>> subsets){
        subsets.add(new ArrayList<>(current));
        for(int i = index; i < nums.length; i++){
            current.add(nums[i]);           
            generateSubsets(i + 1, nums, current, subsets);
            current.remove(current.size()-1);
          
        }
    }
}

I don't understand how this code will run this code at each step. If we take an example input [1,2,3]. I think first I would get the current list [1], then a recursive call and new current list is [1,2] then a new recursive call and current list is [1,2,3] but I am very confused what the next steps would be.

I would really appreciate it if someone could walk me through this process. Especially when the code current.remove(current.size()-1); is hit.

Thank you very much.

Answer 1

Set break points at the lines of interest and use your IDE to debug. Just for a starter you could also put some println statments to follow the recursive code flow:

public static void generateSubsets(int index, int[] nums, List<Integer> current, List<List<Integer>> subsets){
    System.out.println("\nrecursive method called with index: " + index + " current list: " + current);
    subsets.add(new ArrayList<>(current));
    System.out.println("subsets after adding current: " + subsets);
    for(int i = index; i < nums.length; i++){
        System.out.println("entering loop at index: " + index);
        current.add(nums[i]);
        System.out.println("current after adding current index: " + current);
        generateSubsets(i + 1, nums, current, subsets);
        current.remove(current.size()-1);
        System.out.println("current after removing last element back at index: "+ index + " " +current);
    }
}