1670. Design Front Middle Back Queue

Design a queue that supports push and pop operations in the front, middle, and back.

Implement the FrontMiddleBack class:

• FrontMiddleBack() Initializes the queue.
• void pushFront(int val) Adds val to the front of the queue.
• void pushMiddle(int val) Adds val to the middle of the queue.
• void pushBack(int val) Adds val to the back of the queue.
• int popFront() Removes the front element of the queue and returns it. If the queue is empty, return -1.
• int popMiddle() Removes the middle element of the queue and returns it. If the queue is empty, return -1.
• int popBack() Removes the back element of the queue and returns it. If the queue is empty, return -1.

Notice that when there are two middle position choices, the operation is performed on the frontmost middle position choice. For example:

• Pushing 6 into the middle of [1, 2, 3, 4, 5] results in [1, 2, 6, 3, 4, 5].
• Popping the middle from [1, 2, 3, 4, 5, 6] returns 3 and results in [1, 2, 4, 5, 6].

 

Example 1:

Input: ["FrontMiddleBackQueue", "pushFront", "pushBack", "pushMiddle", "pushMiddle", "popFront", "popMiddle", "popMiddle", "popBack", "popFront"] [[], [1], [2], [3], [4], [], [], [], [], []] Output: [null, null, null, null, null, 1, 3, 4, 2, -1] Explanation: FrontMiddleBackQueue q = new FrontMiddleBackQueue(); q.pushFront(1); // [1] q.pushBack(2); // [1, 2] q.pushMiddle(3); // [1, 3, 2] q.pushMiddle(4); // [1, 4, 3, 2] q.popFront(); // return 1 -> [4, 3, 2] q.popMiddle(); // return 3 -> [4, 2] q.popMiddle(); // return 4 -> [2] q.popBack(); // return 2 -> [] q.popFront(); // return -1 -> [] (The queue is empty)

 

Constraints:

• 1 <= val <= 109
• At most 1000 calls will be made to pushFront, pushMiddle, pushBack, popFront, popMiddle, and popBack.

 

 

 

class FrontMiddleBackQueue {
    LinkedList<Integer> left, right;
    int size;

    public FrontMiddleBackQueue() {
        left = new LinkedList<>();
        right = new LinkedList<>();
        size = 0;
    }
    
    public void pushFront(int val) {
        left.addFirst(val);
        if (left.size() > right.size()) {
            right.addFirst(left.removeLast());
        }
        size++;
    }
    
    public void pushMiddle(int val) {
        if (left.size() == right.size()) {
            right.addFirst(val);
        } else {
            left.addLast(val);
        }
        size++;
    }
    
    public void pushBack(int val) {
        if (left.size() < right.size()) {
            left.addLast(right.removeFirst());
        }
        right.addLast(val);
        size++;
    }
    
    public int popFront() {
        if (size == 0) {
            return -1;
        }
        if (left.size() < right.size()) {
            left.addLast(right.removeFirst());
        }
        int val = left.removeFirst();
        size--;
        return val;
    }
    
    public int popMiddle() {
        if (size == 0) {
            return -1;
        }
        int val;
        if (left.size() == right.size()) {
            val = left.removeLast();
        } else {
            val = right.removeFirst();
        }
        size--;
        return val;
    }
    
    public int popBack() {
        if (size == 0) {
            return -1;
        }
        int val = right.removeLast();
        if (left.size() > right.size()) {
            right.addFirst(left.removeLast());
        }
        size--;
        return val;
    }
}

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