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Design Front Middle Back Queue

MediumArrayLinked listDesign
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Medium · linked-list walk solution for Design Front Middle Back Queue. Python, Go, and a step-by-step animation.

01

Problem

Read the prompt, flip through the examples, then steal the idea.

LeetCode 1670. Design Front Middle Back Queue. The copyrighted prompt is on LeetCode. This page is a working linked-list walk solution for this problem: editable input, Python, Go, and a step-by-step animation.

Constraints

  • —Player inputs are capped so the animation stays readable.
  • —Match LeetCode's own constraints when you submit there.

Examples

Input

head =
[1, 2, 3, 4]

Output

[4,3,2,1]

Each node is peeled off cur and pushed onto prev.

02

Solution

Python and Go. The walkthrough below runs this same code.

Approach 1Optimal▶ Animated

Design Front Middle Back Queue — linked-list walk

Three pointers: prev, cur, next. Flip one edge per step. Built for LeetCode 1670. Design Front Middle Back Queue.

You only ever need the next node saved before you rewire cur.next = prev. Walk until cur is nil; prev is the new head.

Algorithm

  1. prev = null, cur = head.
  2. Save next, flip the edge, slide prev and cur forward.
  3. Return prev.
Time
O(n)
Space
O(1)
Solution.py
class Solution:    def designFrontMiddleBackQueue(self, head: list[int]) -> list[int]:        prev: list[int] = []        cur = head        while cur:            nxt = cur[1:]            prev = [cur[0], *prev]            cur = nxt        return prev
Watch this run

03

Interactive walkthrough

Playing “Design Front Middle Back Queue — linked-list walk”. Edit the input, hit Run, then scrub.

Approach: Design Front Middle Back Queue — linked-list walk

Loading animation…