큐

큐는 스택과 마찬가지로 데이터를 쌓아두는 자료구조이다.
스택과 다른점은 스택은 LIFO(후입선출) 큐는 FIFO(선입선출)이다.
먼저 들어온 데이터가 먼저 나가게 된다.

public class IntQueue {
    private int[] que;
    private int capacity;
    private int num;
    private int front;
    private int rear;

    public class EmptyQueueException extends RuntimeException{
        public EmptyQueueException(){}
    }
    public class OverFlowQueueException extends RuntimeException{
        public OverFlowQueueException (){}
    }

    public IntQueue(int len) {
        capacity = len;
        front = rear = num = 0;

        try {
            que = new int[capacity];
        } catch (OutOfMemoryError e) {
            capacity = 0;
        }
    }

    public int enque(int x) throws OverFlowQueueException{
        if(num >= capacity) throw  new OverFlowQueueException();
        que[rear++] = x;
        num ++;
        if(rear == capacity)
            rear = 0;
        return x;
    }

    public int deque() throws EmptyQueueException{
        if(num <= 0) throw new EmptyQueueException();
        int x = que[front++];
        num--;
        if(front == capacity) front = 0;
        return x;
    }

    public int peek() throws EmptyQueueException{
        if(num <= 0) throw new EmptyQueueException();
        return que[front];
    }

    public void clear(){
        num = front = rear = 0;
    }

    public void dump(){
        if(num <= 0) System.out.println("Empty");
        else {
            for (int i = 0; i < num; i++) {
                System.out.print(que[(i + front) % capacity] + " ");
            }
            System.out.println();
        }
    }

    public int indexOf(int x){
        int idx;
        for(int i = 0; i < num; i++){
            idx = (i + front) % capacity;
            if(que[idx] == x){
                return idx;
            }
        }
        return -1;
    }

}

큐 활용 예시

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
       IntQueue que = new IntQueue(10);

       while(true){
           System.out.println("(1) 인큐 (2) 디큐 (3) 피크 (4) 덤프 (5) 인덱스 (6) 초기화 (0) 종료");
           int order = sc.nextInt();
           if(order == 0) break;
           int x;
           switch (order){
               case 1:
                   x = sc.nextInt();
                   que.enque(x);
                   break;
               case 2:
                   x = que.deque();
                   System.out.println(x);
                   break;
               case 3:
                   x = que.peek();
                   System.out.println(x);
                   break;
               case 4:
                   que.dump();
                   break;
               case 5:
                   x = sc.nextInt();
                   int res = que.indexOf(x);
                   System.out.println(res);
                   break;
               case 6:
                   que.clear();
                   break;
               default:
                   break;
           }

       }

    }
}