Given a BST and a key K. If K is not present in the BST, Insert a new Node with a value equal to K into the BST. 
Note: If K is already present in the BST, don't modify the BST.

Example 1:

Input:
     2
   /   \
  1     3
K = 4
Output: 1 2 3 4
Explanation: After inserting the node 4
Inorder traversal will be 1 2 3 4.

Example 2:

Input:
        2
      /   \
     1     3
             \
              6
K = 4
Output: 1 2 3 4 6
Explanation: After inserting the node 4
Inorder traversal of the above tree
will be 1 2 3 4 6.

Your Task:
You don't need to read input or print anything. Your task is to complete the function insert() which takes the root of the BST and Key K as input parameters and returns the root of the modified BST after inserting K. 
Note: The generated output contains the inorder traversal of the modified tree.

Expected Time Complexity: O(Height of the BST).
Expected Auxiliary Space: O(Height of the BST).

Constraints:
1 <= Number of nodes <= 10⁵
1 <= K <= 10⁶