Add new left & right node at depth "d-1".

```
class Solution(object):
def addOneRow(self, root, v, d):
dummy = TreeNode(0)
dummy.left = root
self.dfs(dummy, 0, v, d)
return dummy.left
def dfs(self, root, curr, v, d):
if not root:
return
if curr == d - 1:
lnode, rnode = TreeNode(v), TreeNode(v)
root.left, lnode.left = lnode, root.left
root.right, rnode.right = rnode, root.right
return
self.dfs(root.left, curr+1, v, d)
self.dfs(root.right, curr+1, v, d)
```