1670. Design Front Middle Back Queue

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.

 

Solutions

Solution: Design

• Time complexity: O(n)
• Space complexity: O(n)

 

JavaScript

const FrontMiddleBackQueue = function () {
  this.leftQueue = [];
  this.rightQueue = [];
};

FrontMiddleBackQueue.prototype.balanceQueue = function () {
  const leftSize = this.leftQueue.length;
  const rightSize = this.rightQueue.length;

  if (leftSize > rightSize) {
    this.rightQueue.unshift(this.leftQueue.pop());
  } else if (leftSize + 1 === rightSize - 1) {
    this.leftQueue.push(this.rightQueue.shift());
  }
};

/**
 * @param {number} val
 * @return {void}
 */
FrontMiddleBackQueue.prototype.pushFront = function (val) {
  this.leftQueue.unshift(val);
  this.balanceQueue();
};

/**
 * @param {number} val
 * @return {void}
 */
FrontMiddleBackQueue.prototype.pushMiddle = function (val) {
  this.leftQueue.length === this.rightQueue.length ? this.rightQueue.unshift(val) : this.leftQueue.push(val);
};

/**
 * @param {number} val
 * @return {void}
 */
FrontMiddleBackQueue.prototype.pushBack = function (val) {
  this.rightQueue.push(val);
  this.balanceQueue();
};

/**
 * @return {number}
 */
FrontMiddleBackQueue.prototype.popFront = function () {
  if (!this.leftQueue.length) {
    return this.rightQueue.shift() ?? -1;
  }
  const val = this.leftQueue.shift();

  this.balanceQueue();
  return val;
};

/**
 * @return {number}
 */
FrontMiddleBackQueue.prototype.popMiddle = function () {
  return this.leftQueue.length === this.rightQueue.length ? (this.leftQueue.pop() ?? -1) : this.rightQueue.shift();
};

/**
 * @return {number}
 */
FrontMiddleBackQueue.prototype.popBack = function () {
  const val = this.rightQueue.pop();

  this.balanceQueue();
  return val ?? -1;
};

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