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:

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)

Approach:

C++

#include <bits/stdc++.h>
using namespace std;

class FrontMiddleBackQueue
{
    deque<int> dq;

public:
    FrontMiddleBackQueue()
    {
        dq.clear();
    }

    void pushFront(int val)
    {
        dq.push_front(val);
    }

    void pushMiddle(int val)
    {
        int n = dq.size();
        dq.insert(dq.begin() + n / 2, val);
    }

    void pushBack(int val)
    {
        dq.push_back(val);
    }

    int popFront()
    {
        if (dq.empty())
            return -1;

        int ans = dq.front();
        dq.pop_front();
        return ans;
    }

    int popMiddle()
    {
        if (dq.empty())
            return -1;

        int mid = (dq.size() - 1) / 2;
        int ans = *(dq.begin() + mid);
        dq.erase(dq.begin() + mid);
        return ans;
    }

    int popBack()
    {
        if (dq.empty())
            return -1;

        int ans = dq.back();
        dq.pop_back();
        return ans;
    }
};

int main()
{
    FrontMiddleBackQueue q;
    q.pushFront(1);
    q.pushBack(2);
    q.pushMiddle(3);
    q.pushMiddle(4);
    cout << "[";
    cout << q.popFront() << ", ";
    cout << q.popMiddle() << ", ";
    cout << q.popMiddle() << ", ";
    cout << q.popBack() << ", ";
    cout << q.popFront();
    cout << "]";

    return 0;
}