递归的困惑以及为什么我的列表没有保持更新？

Question

import java.util.*;

public class MyClass {

    public static void main(String args[]) {

        int[] arr = new int[]{2,3,4,5};
        int[] min = new int[]{Integer.MAX_VALUE};
        int tgt = 7;
        List<Integer> lst = new ArrayList<>();
        List<Integer> sol = new ArrayList<>();
        
        recursion(arr, tgt, lst, min, sol);
        
        for (int i = 0; i < sol.size(); i++) {
            System.out.println(sol.get(i));
        }
      
    }
    
    public void recursion(int[] nums, int tgt, List<Integer> lst, int[] minLen, List<Integer> sol) {
        if (tgt < 0) {
            return;
        }
        if (tgt == 0) {
            if (lst.size() <= minLen[0]) {
                minLen[0] = lst.size();
            }
            sol = lst;
            return;
        }
        
        for (int i = 0; i < nums.length; i++) {
            List<Integer> cpy = new ArrayList<>(lst);
            cpy.add(nums[i]);
            recursion(nums, tgt - nums[i], cpy, minLen, sol);
        }
    }
}

The basic logic of what I'm trying to do is, given an array of numbers and a target, I want to return the smallest list of numbers that can be summed up to the target (in this case I'd want to return either {3,4} or {5,2}. I call a recursive function that breaks down my problem until I reach a base case in which I return or do some work on my sol list. lst is there to build up my current list in that recursive path while sol is there to update whenever I find a new path that is the new minimum length. However, what I think is happening is that when sol updates and returns, it becomes reset so sol is empty when it returns to main. I thought the sol list was being added to the Heap and would persist across all the calls when updated (like the minLen array)? Or am I missing something. What would be a way to get around this while maintaining my logic (I don't want to return a value but would rather have some data structure that just updates and that I could just r eturn from my main function).

Answer 1

When you have a code as follows:

    public void recursion(int[] nums, int tgt, List<Integer> lst, int[] minLen, List<Integer> sol) {
        //... removed not relevant code
        sol = lst;

    }

What you are really doing is just assigning sol variable to point to a different object. That doesn't have any affect on the sol variable that you passed as an argument in the main method. That's because java passes arguments by value. In this case value is a reference to the object.

In the recursion method that reference is copied into sol variable (parameter). So, when you assign it a different value ( sol = lst ), you only update value of this one variable, that is local to the function.

You have two solutions here:

1. Just add all values into a list that sol (passed as argument) points to.
2. (better) just return result from within a function and assign it to new variable in main method.

It' usually a bad idea to modify objects passed into the function (unless that's the job of the function - like sort function). It's (almost) always better to return result with a return statement.