set optional part

README.md

@@ -30,7 +30,7 @@ Finally, a few examples of trees will be showcased:
       - [_Question 10._](bst.md#question-10-toc)
     - [_Deletion_](bst.md#deletion-toc)
       - [_Question 11._](bst.md#question-11-toc)
-- [_Unbalanced trees_](bst.md#unbalanced-trees-toc)
+- [_Unbalanced trees_ (_optional_)](bst.md#unbalanced-trees-toc-optional)
   - [_Question 12._](bst.md#question-12-toc)
   - [_Height-balanced BSTs_](avl.md#height-balanced-bsts-toc)
       - [_Question 13._](avl.md#question-13-toc)
@@ -41,7 +41,7 @@ Finally, a few examples of trees will be showcased:
           - [_Question 17._](avl.md#question-17-toc)
       - [_Balancing our BSTs_](avl.md#balancing-our-bsts-toc)
           - [_Question 18._](avl.md#question-18-toc)
-- [_Maze generation and solving_](maze.md#maze-generation-and-solving-toc)
+- [_Maze generation and solving_ (_optional_)](maze.md#maze-generation-and-solving-toc-optional)
   - [_Generating perfect mazes_](maze.md#generating-perfect-mazes-toc)
     - [_Question 19._](maze.md#question-19-toc)
     - [_Question 20._](maze.md#question-20-toc)
@@ -49,7 +49,7 @@ Finally, a few examples of trees will be showcased:
   - [_solving perfect mazes_](maze.md#solving-perfect-mazes-toc)
     - [_Question 22._](maze.md#question-22-toc)
     - [_Question 23._](maze.md#question-23-toc)
-- [_Quad trees_](qtree.md#quad-trees-toc)
+- [_Quad trees_ (_optional_)](qtree.md#quad-trees-toc-optional)
   - [_$n$-body collision simulation_](qtree.md#n-body-collision-simulation-toc)
     - [_Question 24._](qtree.md#question-24-toc)
     - [_Question 25._](qtree.md#question-25-toc)

bst.md

-### Implementing the BST [[toc](README.md#table-of-content)]
+### Implementing the BST [[toc](README.md#table-of-content)] (_optional_)
 
 Now that we have seen what trees are and the basic operations on binary trees,
 it is time to go back to our original problem.

maze.md

-## Maze generation and solving [[toc](README.md#table-of-content)]
+## Maze generation and solving [[toc](README.md#table-of-content)] (_optional_)
 
 In this section, you will have to produce some code that will (1) generate a
 _perfect_ maze and (2) solve the maze.

qtree.md

-## Quad trees [[toc](README.md#table-of-content)]
+## Quad trees [[toc](README.md#table-of-content)] (_optional_)
 
 In this section, we will demonstrate the power of trees by drastically improving
 the performance of an $n$-body simulation.