1. 维护左右两个队列，控制左队列的长度比右队列长，且不超过1
2. pushFront 往左队列的左边添加元素
3. pushMiddle 往左队列的右边或者右队列的左边添加元素
4. 其余同理，可以参照代码

class FrontMiddleBackQueue:def __init__(self):self.llist, self.rlist = [], []def pushFront(self, val: int) -> None:self.llist.insert(0, val)if len(self.llist) > len(self.rlist) + 1:self.rlist.insert(0, self.llist.pop())def pushMiddle(self, val: int) -> None:if len(self.llist) > len(self.rlist):self.rlist.insert(0, self.llist.pop())self.llist.append(val)def pushBack(self, val: int) -> None:self.rlist.append(val)if len(self.rlist) > len(self.llist):self.llist.append(self.rlist.pop(0))def popFront(self) -> int:if len(self.llist) == 0:return -1ans = self.llist.pop(0)if len(self.rlist) > len(self.llist):self.llist.append(self.rlist.pop(0))return ansdef popMiddle(self) -> int:if len(self.llist) == 0:return -1ans = self.llist.pop()if len(self.rlist) > len(self.llist):self.llist.append(self.rlist.pop(0))return ansdef popBack(self) -> int:if len(self.llist) == 0:return -1if len(self.rlist) == 0:return self.llist.pop()ans = self.rlist.pop()if len(self.llist) > len(self.rlist) + 1:self.rlist.insert(0, self.llist.pop())return ans