test.java

public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        //base case
        if(t1==null&&t2==null) return null;
        if(t1==null) return t2;
        if(t2==null) return t1;
        
        //divide and conquer
        TreeNode root = new TreeNode(t1.val+t2.val);
        root.left = mergeTrees(t1.left, t2.left);
        root.right = mergeTrees(t1.right, t2.right);
        
        return root;    
    }