传递新的 ArrayList 与传递 ArrayList 作为函数（Java）中的参数

Question

Please help me understand the difference. Like in first code snippet, I'm passing the updated ArrayList as a parameter in the recursive function in Line 1 & 2. This is giving me wrong answer.

public static void rootToLeafPaths(TreeNode node, ArrayList<Integer> sub){
        if(node == null)
            return;
        sub.add(node.val);
        if(node.left == null && node.right == null){
            list.add(sub);
            return;
        }
        rootToLeafPaths(node.left,sub); //LINE 1
        rootToLeafPaths(node.right,sub); //LINE 2
        sub.remove(sub.size()-1);    
    }

In the second code snippet I'm passing a newly created ArrayList(in Line 3 & 4) which is giving me the correct answer.

public static void rootToLeafPaths(TreeNode node, ArrayList<Integer> sub){
            if(node == null)
                return;
            sub.add(node.val);
            if(node.left == null && node.right == null){
                list.add(sub);
                return;
            }
            rootToLeafPaths(node.left,new ArrayList<Integer> (sub)); // LINE 3
            rootToLeafPaths(node.right,new ArrayList<Integer> (sub)); // LINE 4
            sub.remove(sub.size()-1);    
    }

Help me understand the difference.

This method is supposed to provide the root to leaf paths of a binary tree. Like in first case it is giving me [[1, 2, 4], [1, 2, 4], [1, 2, 4]] as output which is wrong, but when I'm passing a newly created ArrayList each time it is giving me [[1, 2, 4, 6, 8], [1, 2, 5, 7, 9, 10], [1, 3]] as output which is correct

Answer 1

In list.add(sub) you are adding sub List as an element of list List .

In your first snippet, sub always references the same List , which means your output list will contain multiple references to the same List . Hence the wrong answer.

In your second snippet, you create a copy of sub for each recursive call, which means you are adding different List instances to your list .