use isEmpty instead of directly checking N in ArrayStack.pop()
implement linked list queue
implement bag
Rename 1_3 folder to 'ds' (data structures). move the dijkstra arithmetic expression evaluation algorithm to root
fix the import path of queue inside bag
fix the 'stack' import path in dijkstra_arith and change the stack class name to LLStack
add TODO
stack.d: add the whatever you call it, the type parameter, to a usage of the Node class
Add a default case to the switch statement in dijkstra_arith.eval()