class Solution {
    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if(root1 == null && root2 == null) return null;
        else if(root1 == null) return root2;
        else if(root2 == null) return root1;
        
        TreeNode res = new TreeNode(root1.val + root2.val);
        res.left = mergeTrees(root1.left, root2.left);
        res.right = mergeTrees(root1.right, root2.right);
        
        return res;
    }
}