Queue • Algorithms

Reverse First K Elements of Queue

Given an integer K and a queue, reverse the order of the first K elements, leaving the remaining elements in their original relative order.

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1. Isolate and Reverse

Pop the first K elements into a stack. This inherently reverses them. Then pop from the stack and enqueue them back. Now the reversed K elements are at the back of the queue.

2. The Problem

Wait, if they are at the back, the queue is out of order! `1, 2, 3, 4, 5` (k=3) became `4, 5, 3, 2, 1`. The unreversed elements `4, 5` are stuck at the front.

3. The Rotation Fix

To fix this, simply dequeue the remaining `N - K` elements (`4, 5`) and immediately enqueue them back to the rear! This rotates the queue to `3, 2, 1, 4, 5`.

The Queue

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Stack (Capacity K=3)

Code Tracker

:::

1function reverseFirstK(q, k) {

2 let stack = [];

3 for (let i = 0; i < k; i++) {

4 stack.push(q.shift());

5 }

6 while (stack.length > 0) {

7 q.push(stack.pop());

8 }

9 let rem = q.length - k;

10 for (let i = 0; i < rem; i++) {

11 q.push(q.shift());

12 }

13 return q;

14}