Pascal's triangle giving only last row using Java ArrayList

Question

I am trying to generate the pascal's triangle for a lines. For 5 it gives me only the 5th row five times. Dont know why?

public class Solution {
    public ArrayList<ArrayList<Integer>> generate(int a) {

        ArrayList<Integer> internal = new ArrayList<Integer>();
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        for(int i = 0 ; i < a ; i++)
        {
            internal.clear();
            internal.add(1);
            for(int j = 0 ; j < i ; j++ )
            {
                internal.add(internal.get(j) * (i - j)/(j + 1));
            }
            result.add(internal);
        }
        return result;
    }
}

Expected:- [1],[1,1],[1,2,1],[1,3,3,1],[1,4,6,4,1]
Getting:- [1,4,6,4,1],[1,4,6,4,1],[1,4,6,4,1],[1,4,6,4,1],[1,4,6,4,1]

Need reason why this is happening

It's because you're using the same reference repeatedly. Don't use `clear`. Instead start with `new` list. — Paul Boddington, Apr 12 '16 at 19:45
Every time you change your ArrayList "internal" you are changing the whole object that is referenced in your "result" ArrayList. Like Paul said, create a new ArrayList each time and add that to your result. `internal = new ArrayList();` instead of `internal.clear();` — Nick Suwyn, Apr 12 '16 at 19:49

score 0 · Answer 1 · answered Jan 24 '21 at 09:49

import java.util.ArrayList;

public class Solution {
    
    public static void main(String args[]){
        System.out.print(generate(3));
        
    }
    public static ArrayList<ArrayList<Integer>> generate(int a) {

        ArrayList<Integer> internal = new ArrayList<Integer>();
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        for(int i = 0 ; i < a ; i++)
        {
            internal = new ArrayList<Integer>(); //this is the keypoint.
            internal.add(1);
            for(int j = 0 ; j < i ; j++ )
            {
                internal.add(internal.get(j) * (i - j)/(j + 1));
            }
            result.add(internal);
        }
        return result;
    }
}

Output

[1], [1, 1], [1, 2, 1], [1, 3, 3, 1], [1, 4, 6, 4, 1]

Pascal's triangle giving only last row using Java ArrayList

1 Answers1