题目如下：

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.

解题思路：使用两个queue，分别为front和second，始终保证front的长度不小于second，并且与second的长度差不超过2。

代码如下：

class FrontMiddleBackQueue(object):

    def __init__(self):
        self.front = []
        self.second = []

    def pushFront(self, val):
        """
        :type val: int
        :rtype: None
        """
        self.front.insert(0,val)
        if len(self.front) - len(self.second) >= 2:
            self.second.insert(0,self.front.pop(-1))

    def pushMiddle(self, val):
        """
        :type val: int
        :rtype: None
        """
        if len(self.front) == len(self.second):
            self.front.append(val)
        else:
            self.second.insert(0,self.front.pop(-1))
            self.front.append(val)


    def pushBack(self, val):
        """
        :type val: int
        :rtype: None
        """
        self.second.append(val)
        if len(self.front) < len(self.second):
            self.front.append(self.second.pop(0))


    def popFront(self):
        """
        :rtype: int
        """
        if len(self.front) == 0:
            return -1
        val = self.front.pop(0)
        if len(self.front) < len(self.second):
            self.front.append(self.second.pop(0))
        return val

    def popMiddle(self):
        """
        :rtype: int
        """
        if len(self.front) == 0:
            return -1
        val = self.front.pop(-1)
        if len(self.front) < len(self.second):
            self.front.append(self.second.pop(0))
        return val


    def popBack(self):
        """
        :rtype: int
        """
        if len(self.second) == 0 and len(self.front) == 0:
            return -1
        elif len(self.second) == 0 and len(self.front) != 0:
            return self.front.pop(-1)
        val = self.second.pop(-1)
        if len(self.front) - len(self.second) >= 2:
            self.second.insert(0,self.front.pop(-1))
        return val
        


# 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()