Merge Two Binary Trees — Recursive Overlay

# Python
def mergeTrees(root1, root2):
    if not root1: return root2
    if not root2: return root1
    root1.val += root2.val
    root1.left = mergeTrees(root1.left, root2.left)
    root1.right = mergeTrees(root1.right, root2.right)
    return root1

// Java
public TreeNode mergeTrees(TreeNode r1, TreeNode r2) {
    if (r1==null) return r2; if (r2==null) return r1;
    r1.val+=r2.val;
    r1.left=mergeTrees(r1.left,r2.left);
    r1.right=mergeTrees(r1.right,r2.right);
    return r1;
}