Data Structures, Algorithms, & Applications in Java
Chapter 12, Exercise 57

Any sequence which always combines sets that have the same number of elements will do. For example, the sequence union(1,2), union(3,4), union(1,3) results in a tree of 4 elements and height 3. The sequence union(5,6), union(7,8), union(5,7) also results in a height 3 tree with 4 elements. Performing the operation union(1,5) now results in a height 4 tree with 8 elements.