bstsearch.swift

func searchBST(_ root: TreeNode?, _ val: Int) -> TreeNode? {
        
    /**
    Without a root, nothing to find.
    This is the exist condition of our recursion.
    */
    guard let node = root else {
        return nil
    }

    /**
    First we need to check if the 
    node.val is the target value.
    */
    if node.val == val {

        return node 

    /**
    Next we need to check if the target value is larger
    or smaller than node's.val...
    */
    } else if node.val > val {

        // ... and continue down the recursion ...
        return searchBST(node.left, val)

    } else {

        // ... and pass the proper sub tree.
        return searchBST(node.right, val)
    }
}