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].

class FrontMiddleBackQueue {
    deque<int> f, b;
    void balance() {
        int s = f.size() + b.size();
        if(s & 1) {
            if(f.size() > b.size()) {
                while(f.size() > b.size()) {
                    b.push_front(f.back());
                    f.pop_back();
                }
            } else if(b.size() > f.size()) {
                while(f.size() + 1 != b.size()) {
                    f.push_back(b.front());
                    b.pop_front();
                }
            }
        } else {
            if(f.size() != b.size()) {
                if(f.size() > b.size()) {
                    while(f.size() != b.size()) {
                        b.push_front(f.back());
                        f.pop_back();
                    }
                } else {
                    while(f.size() != b.size()) {
                        f.push_back(b.front());
                        b.pop_front();
                    }
                }
            }
        }
    }
public:
    FrontMiddleBackQueue() {}

    void pushFront(int val) {
        f.push_front(val);
        balance();
    }

    void pushMiddle(int val) {
        f.push_back(val);
        balance();
    }

    void pushBack(int val) {
        b.push_back(val);
        balance();
    }

    int popFront() {
        int res = -1;
        if(!f.empty()) {
            res = f.front();
            f.pop_front();
        } else if(!b.empty()) {
            res = b.front();
            b.pop_front();
        }
        balance();
        return res;
    }

    int popMiddle() {
        int s = f.size() + b.size();
        int res = -1;
        if(s != 0) {
            if(s & 1) {
                if(!b.empty()) {
                    res = b.front();
                    b.pop_front();
                } else if(!f.empty()) {
                    res = f.back();
                    b.pop_back();
                }
            } else {
                if(!f.empty()) {
                    res = f.back();
                    f.pop_back();
                } else if(!b.empty()) {
                    res = b.front();
                    b.pop_front();
                }
            }
            balance();
        }
        return res;
    }

    int popBack() {
        int res = -1;
        if(!b.empty()) {
            res = b.back();
            b.pop_back();
        } else if(!f.empty()) {
            res = f.back();
            f.pop_back();
        }
        balance();
        return res;
    }
};

/**
 * Your FrontMiddleBackQueue object will be instantiated and called as such:
 * FrontMiddleBackQueue* obj = new FrontMiddleBackQueue();
 * obj->pushFront(val);
 * obj->pushMiddle(val);
 * obj->pushBack(val);
 * int param_4 = obj->popFront();
 * int param_5 = obj->popMiddle();
 * int param_6 = obj->popBack();
 */