start                 : 20, 30, 40, 50, 60, 70, 80   [height 2]
contains(40) = true, contains(45) = false
minimum = 20, maximum = 80

case 1, delete 20     : 30, 40, 50, 60, 70, 80   [height 2]
case 2, delete 30     : 40, 50, 60, 70, 80   [height 2]
case 3, delete 50     : 40, 60, 70, 80   [height 2]
