disjoint set data structure

Question

this is the code for findind disjoint sets

    class disjoint_sets {
      struct disjoint_set {
      size_t parent;
      unsigned rank;
      disjoint_set(siz         


        

Answer 1

Because in this case - you add one tree is a "sub tree" of the other - which makes the original subtree increase its size.

Have a look at the following example:

1           3
|           |
2           4

In the above, the "rank" of each tree is 2.
Now, let's say 1 is going to be the new unified root, you will get the following tree:

    1
   / \
  /   \
 3     2
 |
 4

after the join the rank of "1" is 3, rank_old(1) + 1 - as expected.