1670. Design Front Middle Back Queue

Problem Link

Description

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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.

Solution

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C++

class FrontMiddleBackQueue {
    deque<int> a, b;
 
    void a2b() {
        if (a.size() <= b.size()) return;
        b.push_front(a.back());
        a.pop_back();
    }
 
    void b2a() {
        if (b.size() <= a.size() + 1) return;
        a.push_back(b.front());
        b.pop_front();
    }
 
public:
    FrontMiddleBackQueue() {
        
    }
    
    void pushFront(int val) {
        a.push_front(val);
        a2b();
    }
    
    void pushMiddle(int val) {
        a.push_back(val);
        a2b();
    }
    
    void pushBack(int val) {
        b.push_back(val);
        b2a();
    }
    
    int popFront() {
        if (a.empty() && b.empty()) return -1;
 
        int ans;
 
        if (a.empty()) {
            ans = b.front();
            b.pop_front();
        } else {
            ans = a.front();
            a.pop_front();
            b2a();
        }
 
        return ans;
    }
    
    int popMiddle() {
        if (a.empty() && b.empty()) return -1;
 
        int ans;
 
        if (a.size() == b.size()) {
            ans = a.back();
            a.pop_back();
        } else {
            ans = b.front();
            b.pop_front();
        }
 
        return ans;
    }
    
    int popBack() {
        if (a.empty() && b.empty()) return -1;
 
        int ans = b.back();
        b.pop_back();
        a2b();
 
        return ans;
    }
};
 
/**
 * 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();
 */

Python3

class FrontMiddleBackQueue:
 
    def __init__(self):
        self.A = collections.deque()
        self.B = collections.deque()
        
    def pushFront(self, val: int) -> None:
        self.A.appendleft(val)
        self.balance()
 
    def pushMiddle(self, val: int) -> None:
        if len(self.A) > len(self.B):
            self.B.appendleft(self.A.pop())
        self.A.append(val)
 
    def pushBack(self, val: int) -> None:
        self.B.append(val)
        self.balance()
 
    def popFront(self) -> int:
        val = self.A.popleft() if self.A else -1
        self.balance()
        return val
        
    def popMiddle(self) -> int:
        val = self.A.pop() if self.A else -1
        self.balance()
        return val
 
    def popBack(self) -> int:
        val = self.B.pop() if self.B else self.A.pop() if self.A else -1
        self.balance()
        return val
        
    def balance(self) -> None:
        if len(self.A) > len(self.B) + 1:
            self.B.appendleft(self.A.pop())
        
        if len(self.A) < len(self.B):
            self.A.append(self.B.popleft())
        
 
 
# Your FrontMiddleBackQueue object will be instantiated and called as such:
# obj = FrontMiddleBackQueue()
# obj.pushFront(val)
# obj.pushMiddle(val)
# obj.pushBack(val)
# param_4 = obj.popFront()
# param_5 = obj.popMiddle()
# param_6 = obj.popBack()