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1670. Design Front Middle Back Queue

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 (Rust)

1. Solution

use std::collections::VecDeque;structFrontMiddleBackQueue{front_half:VecDeque<i32>,back_half:VecDeque<i32>,}/** * `&self` means the method takes an immutable reference. * If you need a mutable reference, change it to `&mut self` instead. */implFrontMiddleBackQueue{fnnew() -> Self{Self{front_half:VecDeque::new(),back_half:VecDeque::new(),}}fnpush_front(&mutself,val:i32){self.front_half.push_front(val);ifself.front_half.len() > self.back_half.len(){self.back_half.push_front(self.front_half.pop_back().unwrap());}}fnpush_middle(&mutself,val:i32){ifself.front_half.len() < self.back_half.len(){self.front_half.push_back(val);}else{self.back_half.push_front(val);}}fnpush_back(&mutself,val:i32){self.back_half.push_back(val);ifself.front_half.len() + 1 < self.back_half.len(){self.front_half.push_back(self.back_half.pop_front().unwrap());}}fnpop_front(&mutself) -> i32{ifself.front_half.len() < self.back_half.len(){self.front_half.push_back(self.back_half.pop_front().unwrap());}self.front_half.pop_front().unwrap_or(-1)}fnpop_middle(&mutself) -> i32{ifself.front_half.len() < self.back_half.len(){self.back_half.pop_front().unwrap_or(-1)}else{self.front_half.pop_back().unwrap_or(-1)}}fnpop_back(&mutself) -> i32{ifself.front_half.len() == self.back_half.len(){self.back_half.push_front(self.front_half.pop_back().unwrap_or(-1));}self.back_half.pop_back().unwrap()}}/** * Your FrontMiddleBackQueue object will be instantiated and called as such: * let obj = FrontMiddleBackQueue::new(); * obj.push_front(val); * obj.push_middle(val); * obj.push_back(val); * let ret_4: i32 = obj.pop_front(); * let ret_5: i32 = obj.pop_middle(); * let ret_6: i32 = obj.pop_back(); */
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