To help you finish these problems before the midterm, here's the recommended order to solve problems. But you can still have your own solving order.
Easy: [13018, 13420]
Medium: [13015, 13020, 13673]
Hard: [13716]
Techniques may be used in problems:
13015: DFS
13018: sort method (bubble sort, insertion sort, qsort, etc.)
13020: recursion
13420: DFS
13673: recursion
13716: BFS (Problem has been designed well so DFS is also acceptable)
About DFS and BFS: