test.java

public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if(t1==null) return t2;
        if(t2==null) return t1;
        
        //use st1 for t1 and st2 for t2. You can use one stack as well where you store pairs
        Stack<TreeNode> st1 = new Stack<>();
        Stack<TreeNode> st2 = new Stack<>();
        
        //invariant: neither n1 or n2 can be null
        st1.push(t1);
        st2.push(t2);
        while(!st1.isEmpty()&&!st2.isEmpty())
        {
            TreeNode n1 = st1.pop();
            TreeNode n2 = st2.pop();
            n1.val+=n2.val;
            
            //handle left subtree
            if(n1.left==null)
            {
                n1.left = n2.left;
            }
            else if(n2.left!=null) //keep the invariant
            {
                st1.push(n1.left);
                st2.push(n2.left);
            }
            
            //handle right subtree
            if(n1.right==null)
            {
                n1.right = n2.right;
            }
            else if(n2.right!=null) //keep the invariant
            {
                st1.push(n1.right);
                st2.push(n2.right);
            }
        }
        return t1;
    }