merging 2 binary trees- Easy

Question

I am trying to merge 2 binary trees, without worrying about making the resultant tree balanced. Here is my solution which does not work. Why are the Treenode ans and head set to 0 when they come back from merge functions. as i understand since TreeNode is not primitive type, head which points to ans should be updated with the resultant tree after call to merge function https://leetcode.com/problems/merge-two-binary-trees/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        
        
        
        TreeNode ans = new TreeNode();
        TreeNode head = ans;
        
        if(root1!=null && root2==null)
            return root1;
        else if (root1==null && root2!=null)
            return root2;
        else if (root1!=null &&root2!=null)
            merge (root1, root2,ans) ;   
            
            return head;
            
        
    }
    
    void merge (TreeNode root1, TreeNode root2,TreeNode ans)
    {
         if(root1!=null && root2==null)
            root2 = new TreeNode(0);
        
         else if (root1==null && root2!=null)
            root1 = new TreeNode(0);
         else if(root1==null &&root2==null)
         return;
        
            ans = new TreeNode(root1.val+root2.val); 
              merge(root1.left,root2.left,ans.left);
                merge(root1.right,root2.right,ans.right);          
    }
    
}

Answer 1

The problem is that the variables head and ans are not going to be updated like you expect them to. Java is pass-by-value as explained here. This means that ans is a reference that you're passing the value of to the function merge and then overwriting over there.

You are also re-assigning the value of ans with every recursive call of merge. To return the correct value you would need to return the value assigned to that variable on the very first call.

Hope this is helpful.

Answer 2

Java passes references by value. Inside the method if you change the value of an attribute of the object that will work. But if you assign the pointer to a new object it will not chnage the original object