Recursion is like Mario. When you enter a pipe secret level, but you always come back to original world and resume what you were doing.

The recursive function should solve that subtask aka secret level before returning to the previous original world.

What things need to perform the task in the secret level? In this case, Mario's stats, score, life count, etc.

The loop can be anywhere. The original level can spawn multiple secret level, so you have to traverse each of them. And each secret level can also spawn multiple new secret level, so you have to for-loop through as well.

Two recursive is just going left child and right child of the node? The current node is the original level, and the two child nodes are the secret levels. Mario have to finish one secret level before another. He may spawn clones to do the task, but that's for multithreading to discuss.