Traverse tree using Breath-First traversal.
Implement breath-first traversal - visit every node on a level before going to a lower level.
// ---------Tree------------
//
// A
// / \
// B C
//
// --------------------------
val tree = BinarySearchTree<Char>()
tree.add('A')
tree.add('B')
tree.add('C')
tree.traverseBreathFirst() // listOf('A', 'B', 'C')