A queue removes items in the order they arrived: first in, first out (FIFO). Think of a line for a restroom—the first person to join is the first served. A stack does the opposite, removing the most recently added item first. section 2.2
In breadth-first search, a queue holds the frontier: states discovered but not yet checked. Adding successors at the back and taking the oldest state from the front checks nearby states before more distant ones. Replacing a depth-first search’s stack with a queue changes its search order without changing the basic search loop. section 2.2
public class Queue<T> {
private var container: [T] = [T]()
public var isEmpty: Bool { return container.isEmpty }
public func push(thing: T) { container.append(thing) }
public func pop() -> T { return container.removeFirst() }
}
The Swift implementation appends with push() and removes the first Array element with pop(); isEmpty lets the search loop check before popping. section 2.2
Finding a route
Starting in Boston, breadth-first search checks stations one stop away, then two stops away, and so on. When it reaches New York, it has already checked routes with fewer stops. The same queue-driven approach finds a shortest solution through the example maze. section 2.2
