Adding one row to a Binary Tree

Love tree problems, once you get it right, there are usually very few corner cases (other than the d=1, which was mentioned in the problem description anyways). Here it is, and code is down below.

Song that goes along well with this problem: https://www.youtube.com/watch?v=7HHBFkdHyRU&list=PL9LM05lDUoXqVDYM5MIM4owPiYLCm_Ucm&index=385

Cheers, ACC.

https://leetcode.com/problems/add-one-row-to-tree/

623. Add One Row to Tree
Medium
Given the root of a binary tree, then value v and depth d, you need to add a row of nodes with value v at the given depth d. The root node is at depth 1.
The adding rule is: given a positive integer depth d, for each NOT null tree nodes N in depth d-1, create two tree nodes with value v as N's left subtree root and right subtree root. And N's original left subtree should be the left subtree of the new left subtree root, its original right subtree should be the right subtree of the new right subtree root. If depth d is 1 that means there is no depth d-1 at all, then create a tree node with value v as the new root of the whole original tree, and the original tree is the new root's left subtree.
Example 1:
Input: 
A binary tree as following:
       4
     /   \
    2     6
   / \   / 
  3   1 5   

v = 1

d = 2

Output: 
       4
      / \
     1   1
    /     \
   2       6
  / \     / 
 3   1   5   

Example 2:
Input: 
A binary tree as following:
      4
     /   
    2    
   / \   
  3   1    

v = 1

d = 3

Output: 
      4
     /   
    2
   / \    
  1   1
 /     \  
3       1
Note:
  1. The given d is in range [1, maximum depth of the given tree + 1].
  2. The given binary tree has at least one tree node.


public class Solution
{
    public TreeNode AddOneRow(TreeNode root, int v, int d)
    {
        //Special case
        if (d == 1) 
        {
            TreeNode newRoot = new TreeNode(v);
            newRoot.left = root;
            return newRoot;
        }

        AddOneRow(root, v, 1, d);
        return root;
    }

    private void AddOneRow(TreeNode node, int v, int currentd, int d)
    {
        if (node == null) return;
        if (currentd == d - 1)
        {
            AddNode(node, v);
            return;
        }
        AddOneRow(node.left, v, currentd + 1, d);
        AddOneRow(node.right, v, currentd + 1, d);
    }

    private void AddNode(TreeNode node, int v)
    {
        if (node == null) return;

        TreeNode leftTemp = node.left;
        node.left = new TreeNode(v);
        node.left.left = leftTemp;

        TreeNode rightTemp = node.right;
        node.right = new TreeNode(v);
        node.right.right = rightTemp;
    }
}

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