Queue.swift

struct Queue<T> {
    private var elements: [T] = []
    
    // Add an element to the back of the queue
    mutating func enqueue(_ element: T) {
        elements.append(element)
    }
    
    // Remove and return the front element
    mutating func dequeue() -> T? {
        return elements.isEmpty ? nil : elements.removeFirst()
    }
    
    // Return the front element without removing it
    func peek() -> T? {
        return elements.first
    }
    
    // Check if the queue is empty
    func isEmpty() -> Bool {
        return elements.isEmpty
    }
    
    // Return the size of the queue
    func size() -> Int {
        return elements.count
    }
}

// Example usage:
var queue = Queue<Int>()

queue.enqueue(1)
queue.enqueue(2)
queue.enqueue(3)

print(queue.dequeue()) // Prints: Optional(1)
print(queue.peek())    // Prints: Optional(2)
print(queue.size())    // Prints: 2
print(queue.isEmpty()) // Prints: false