README.md

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;

struct FrontMiddleBackQueue {
    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.
 */
impl FrontMiddleBackQueue {
    fn new() -> Self {
        Self {
            front_half: VecDeque::new(),
            back_half: VecDeque::new(),
        }
    }

    fn push_front(&mut self, val: i32) {
        self.front_half.push_front(val);
        if self.front_half.len() > self.back_half.len() {
            self.back_half
                .push_front(self.front_half.pop_back().unwrap());
        }
    }

    fn push_middle(&mut self, val: i32) {
        if self.front_half.len() < self.back_half.len() {
            self.front_half.push_back(val);
        } else {
            self.back_half.push_front(val);
        }
    }

    fn push_back(&mut self, val: i32) {
        self.back_half.push_back(val);
        if self.front_half.len() + 1 < self.back_half.len() {
            self.front_half
                .push_back(self.back_half.pop_front().unwrap());
        }
    }

    fn pop_front(&mut self) -> i32 {
        if self.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)
    }

    fn pop_middle(&mut self) -> i32 {
        if self.front_half.len() < self.back_half.len() {
            self.back_half.pop_front().unwrap_or(-1)
        } else {
            self.front_half.pop_back().unwrap_or(-1)
        }
    }

    fn pop_back(&mut self) -> i32 {
        if self.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();
 */