Java Community | Help. Code. Learn.•2y ago

Cannot understand this behaviour of java when adding a list to a list of lists.

https://leetcode.com/problems/subsets/submissions/1236349921
this is the leetcode ques i was solving
as you can see the ans list which is supposed to be a subset of the set is being formed and is printed in the stdout just before it was added to the fin global string which is ultimately returned as the answer.
on further debugging its adding the current string n number of times where n is the array index of the power set array
so in the last iteration its adding a null set n(8) times.
But when i pass a clone of the list in function call, the program works as intended
Any explanation?

JavaBot•4/19/24, 6:42 AM

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webbdays•4/19/24, 10:47 AM

code?

JavaBot•4/19/24, 3:50 PM

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Wwebbdays code?

lucifer7303OP•4/19/24, 3:51 PM

Its in the link

lucifer7303OP•4/19/24, 3:51 PM

Its a leetcode submission with the code, the stdout and the return value

Llucifer7303 Its in the link

Kyo-chan•4/19/24, 3:52 PM

No, it says "submission does not exist"

lucifer7303OP•4/19/24, 3:53 PM

https://leetcode.com/problems/subsets/submissions/1236349921

KKyo-chan No, it says "submission does not exist"

lucifer7303OP•4/19/24, 3:54 PM

sorry, it mustve expired, updated the link

Kyo-chan•4/19/24, 3:54 PM

Same result. I'm not a leetcode expert, but maybe it doesn't allow you to share code

lucifer7303OP•4/19/24, 3:55 PM

wait lemme share a screenshot

Kyo-chan•4/19/24, 3:55 PM

yeah, no

Kyo-chan•4/19/24, 3:55 PM

Computers are capable of copying text. Do that instead

dan1st | DanielAPP•4/19/24, 3:55 PM

javaclass Solution {
   List<List<Integer>> fin=new ArrayList<>();
    void helper(List<Integer> ans,int[] nums,int i){
        // System.out.println(ans.toString());
        if(i>=nums.length){
            System.out.print(ans.toString()+" ");
            // System.out.println(fin.toString());
            fin.add(ans);
            return;
        }
        ans.add(nums[i]);
        helper(ans,nums,i+1);
        ans.removeLast();
        helper(ans,nums,i+1);

    }
    public List<List<Integer>> subsets(int[] nums) {
    //   List<List<Integer>> fin=new ArrayList<>();
      helper(new ArrayList<>(),nums,0);
      return fin;  
    }
}

javaclass Solution {
   List<List<Integer>> fin=new ArrayList<>();
    void helper(List<Integer> ans,int[] nums,int i){
        // System.out.println(ans.toString());
        if(i>=nums.length){
            System.out.print(ans.toString()+" ");
            // System.out.println(fin.toString());
            fin.add(ans);
            return;
        }
        ans.add(nums[i]);
        helper(ans,nums,i+1);
        ans.removeLast();
        helper(ans,nums,i+1);

    }
    public List<List<Integer>> subsets(int[] nums) {
    //   List<List<Integer>> fin=new ArrayList<>();
      helper(new ArrayList<>(),nums,0);
      return fin;  
    }
}

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lucifer7303OP•4/19/24, 3:56 PM

i remember there being a rule for not posting the whole code somewhere in the server, maybe im confused with the rules of another server

lucifer7303OP•4/19/24, 3:56 PM

the subset() function is called by the main leetcode driver code

Kyo-chan•4/19/24, 3:57 PM

It's not so much a rule as a limitation of discord, and it's not like a screenshot would do anything better

lucifer7303OP•4/19/24, 3:57 PM

lucifer7303OP•4/19/24, 3:57 PM

the input,output(return of the subset() function) and the stdout

lucifer7303OP•4/19/24, 3:58 PM

i tried taking the fin ans list as a parameter for the helper function but that doesnt help

Llucifer7303 Click to see attachment

lucifer7303OP•4/19/24, 3:59 PM

as you can see the ans list is being formed and printed but when i do fin.add(ans) it just adds it n number of times, with n being its (index+1) in the fin list

Kyo-chan•4/19/24, 4:02 PM

Obviously add() is called more than once

lucifer7303OP•4/19/24, 4:02 PM

what?

lucifer7303OP•4/19/24, 4:02 PM

doesnt it add at the end of the list

Kyo-chan•4/19/24, 4:02 PM

You said that when you call add(), it adds n number of times

lucifer7303OP•4/19/24, 4:03 PM

im adding a list at the end of a list of lists

Kyo-chan•4/19/24, 4:03 PM

No, it's just that add() was called n times

lucifer7303OP•4/19/24, 4:03 PM

but how

lucifer7303OP•4/19/24, 4:03 PM

the prob and confusing part is

lucifer7303OP•4/19/24, 4:03 PM

when i pass the clone of the ans list

lucifer7303OP•4/19/24, 4:03 PM

then this code works perfectly

Kyo-chan•4/19/24, 4:04 PM

Well add() is part of your finishing logic of the recursion

Llucifer7303 when i pass the clone of the ans list

Kyo-chan•4/19/24, 4:04 PM

I'm not sure why that's weird. You're supposed to do exactly that

dan1st | DanielAPP•4/19/24, 4:04 PM

javajavaclass Solution {
   List<List<Integer>> fin=new ArrayList<>();
    void helper(List<Integer> ans,int[] nums,int i){
        // System.out.println(ans.toString());
        if(i>=nums.length){
            //System.out.print(ans.toString()+" ");
            // System.out.println(fin.toString());
            fin.add(ans);
            return;
        }
        ans.add(nums[i]);
        helper(new ArrayList(ans),nums,i+1);
        ans.removeLast();
        helper(new ArrayList(ans),nums,i+1);

    }
    public List<List<Integer>> subsets(int[] nums) {
    //   List<List<Integer>> fin=new ArrayList<>();
      helper(new ArrayList<>(),nums,0);
      return fin;  
    }
}

javajavaclass Solution {
   List<List<Integer>> fin=new ArrayList<>();
    void helper(List<Integer> ans,int[] nums,int i){
        // System.out.println(ans.toString());
        if(i>=nums.length){
            //System.out.print(ans.toString()+" ");
            // System.out.println(fin.toString());
            fin.add(ans);
            return;
        }
        ans.add(nums[i]);
        helper(new ArrayList(ans),nums,i+1);
        ans.removeLast();
        helper(new ArrayList(ans),nums,i+1);

    }
    public List<List<Integer>> subsets(int[] nums) {
    //   List<List<Integer>> fin=new ArrayList<>();
      helper(new ArrayList<>(),nums,0);
      return fin;  
    }
}

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lucifer7303OP•4/19/24, 4:05 PM

this little difference in the function call of helper

lucifer7303OP•4/19/24, 4:05 PM

fixes the code

KKyo-chan I'm not sure why that's weird. You're supposed to do exactly that

lucifer7303OP•4/19/24, 4:05 PM

but why cant i pass the original list

lucifer7303OP•4/19/24, 4:05 PM

it worked in c++ when a friend of mine did it

Kyo-chan•4/19/24, 4:05 PM

.... Because you're constantly modifying it

lucifer7303OP•4/19/24, 4:05 PM

he didnt pass its clone

Kyo-chan•4/19/24, 4:06 PM

I imagine he used a list class directly rather than a pointer to a list class

lucifer7303OP•4/19/24, 4:07 PM

iirc no, i copied the code almost the same to replicate the code

Kyo-chan•4/19/24, 4:07 PM

So what? They're different languages, and notably C++ does have stuff like pointers

lucifer7303OP•4/19/24, 4:07 PM

im weak at pointers so maybe i didnt understand the actual happening

Kyo-chan•4/19/24, 4:08 PM

C++ with typically copy objects constantly rather than point to them, unless you tell it otherwise

lucifer7303OP•4/19/24, 4:08 PM

but the thing is the ans list is being formed correctly in the stdout just before adding it to fin

Llucifer7303 Click to see attachment

lucifer7303OP•4/19/24, 4:09 PM

im talking about the stdout here

lucifer7303OP•4/19/24, 4:09 PM

its not like the ans list(the first parameter) isnt being formed correctly

Kyo-chan•4/19/24, 4:09 PM

Because stdout copies the current content of the list and prints it right here right now. It doesn't matter that you change the list later

lucifer7303OP•4/19/24, 4:10 PM

javaclass Solution { List<List<Integer>> fin=new ArrayList<>(); void helper(List<Integer> ans,int[] nums,int i){ // System.out.println(ans.toString()); if(i>=nums.length){ System.out.print(ans.toString()+" "); // System.out.println(fin.toString()); fin.add(ans); return; } ans.add(nums[i]); helper(ans,nums,i+1); ans.removeLast(); helper(ans,nums,i+1); } public List<List<Integer>> subsets(int[] nums) { // List<List<Integer>> fin=new ArrayList<>(); helper(new ArrayList<>(),nums,0); return fin; } }

javajavaclass Solution { List<List<Integer>> fin=new ArrayList<>(); void helper(List<Integer> ans,int[] nums,int i){ // System.out.println(ans.toString()); if(i>=nums.length){ //System.out.print(ans.toString()+" "); // System.out.println(fin.toString()); fin.add(ans); return; } ans.add(nums[i]); helper(new ArrayList(ans),nums,i+1); ans.removeLast(); helper(new ArrayList(ans),nums,i+1); } public List<List<Integer>> subsets(int[] nums) { // List<List<Integer>> fin=new ArrayList<>(); helper(new ArrayList<>(),nums,0); return fin; } }

Cannot understand this behaviour of java when adding a list to a list of lists.

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