Queue • Core Algorithms

Queue Reversal

Because Queues are strictly FIFO, you cannot iterate backward. To reverse a Queue, you must leverage the LIFO property of a Stack.

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The Dilemma

You have elements `1, 2, 3` in a queue. You need them to be `3, 2, 1`. But the queue only lets you remove `1` first!

Phase 1: Empty into Stack

Dequeue every element and push it onto a Stack. Since `1` is pushed first, it goes to the absolute bottom of the stack. `3` goes to the top.

Phase 2: Refill Queue

Pop from the Stack and enqueue back into the Queue. Because `3` is on top, it gets popped and enqueued first! The LIFO nature of the stack naturally reversed the entire set.

The Queue

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Front (Dequeue here)Rear (Enqueue here)

Auxiliary Stack

Code Tracker

:::

1function reverseQueue(q) {

2 let stack = [];

3 while (q.length > 0) {

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

5 }

6 while (stack.length > 0) {

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

8 }

9 return q;

10}