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1670. Design Front Middle Back Queue

Difficulty Topics

Description

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

design-front-middle-back-queue.py
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()