Day17-Apr-Increasing_OST

class Solution {

    func increasingBST(_ root: TreeNode?) -> TreeNode? {

        // increasingOST : increasing order search tree.
        var increasingOST: TreeNode? = TreeNode(-1)
        var current_node = increasingOST

        // inner method
        func inorderTraversal(_ root: TreeNode?) {
            guard let root = root else {
                return
            }
            // traverse the left subtree
            inorderTraversal(root.left)

            // create a new node using current root node val
            // assign as a right child to current node of\
            // the increasing order search tree..
            current_node?.right = TreeNode(root.val)
            //make it looking-up for the next right node.
            current_node = current_node!.right

            // traverse the right subtree
            inorderTraversal(root.right)
        }

        inorderTraversal(root)

        return increasingOST?.right
    }
}