Design front middle back queue

Array Linked List Design Queue Doubly-Linked List Data Stream

56.9% acceptance

Feb 23, 2026

815

114

Design a queue that supports push and pop operations in the front, middle, and back. 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 and returns it. If empty, return -1. int popMiddle() Removes the middle element and returns it. If empty, return -1. int popBack() Removes the back element and returns it. If empty, return -1. When there are two middle positions, use the frontmost. Example: ["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]

Solution

Rust

Time O(2^n)

Space O(n)

LeetCode

solution.rs

pub struct FrontMiddleBackQueue {
  data: Vec<i32>,
}

impl FrontMiddleBackQueue {
  pub fn new() -> Self {
    FrontMiddleBackQueue { data: Vec::new() }
  }

  pub fn push_front(&mut self, val: i32) {
    self.data.insert(0, val);
  }

  pub fn push_middle(&mut self, val: i32) {
    let mid = self.data.len() / 2;
    self.data.insert(mid, val);
  }

  pub fn push_back(&mut self, val: i32) {
    self.data.push(val);
  }

  pub fn pop_front(&mut self) -> i32 {
    if self.data.is_empty() { -1 } else { self.data.remove(0) }
  }

  pub fn pop_middle(&mut self) -> i32 {
    if self.data.is_empty() {
      -1
    } else {
      let mid = (self.data.len() - 1) / 2;
      self.data.remove(mid)
    }
  }

  pub fn pop_back(&mut self) -> i32 {
    self.data.pop().unwrap_or(-1)
  }
}