# Reversing a Queue

Give an algorithm for reversing a queue Q. Only following standard operations are allowed on queue.

1. enqueue(x) : Add an item x to rear of queue.
2. dequeue() : Remove an item from front of queue.
3. empty() : Checks if a queue is empty or not.

Examples:

```Input :  Q = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
Output : Q = [100, 90, 80, 70, 60, 50, 40, 30, 20, 10]

Input : [1, 2, 3, 4, 5]
Output : [5, 4, 3, 2, 1]
```

## Recommended: Please solve it on PRACTICE first, before moving on to the solution.

Approach: For reversing the queue one approach could be to store the elements of the queue in a temporary data structure in a manner such that if we re-insert the elements in the queue they would get inserted in reverse order. So now our task is to choose such data-structure which can serve the purpose. According to the approach, the data-structure should have the property of ‘LIFO’ as the last element to be inserted in the data structure should actually be the first element of the reversed queue. The stack could help in approaching this problem. This will be a two-step process:

1. Pop the elements from the queue and insert into the stack. (Topmost element of the stack is the last element of the queue)
2. Pop the elements of the stack to insert back into the queue. (The last element is the first one to be inserted into the queue)

## C++

 `// CPP program to reverse a Queue ` `#include ` `using` `namespace` `std; ` ` `  `// Utility function to print the queue ` `void` `Print(queue<``int``>& Queue) ` `{ ` `    ``while` `(!Queue.empty()) { ` `        ``cout << Queue.front() << ``" "``; ` `        ``Queue.pop(); ` `    ``} ` `} ` ` `  `// Function to reverse the queue ` `void` `reverseQueue(queue<``int``>& Queue) ` `{ ` `    ``stack<``int``> Stack; ` `    ``while` `(!Queue.empty()) { ` `        ``Stack.push(Queue.front()); ` `        ``Queue.pop(); ` `    ``} ` `    ``while` `(!Stack.empty()) { ` `        ``Queue.push(Stack.top()); ` `        ``Stack.pop(); ` `    ``} ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``queue<``int``> Queue; ` `    ``Queue.push(10); ` `    ``Queue.push(20); ` `    ``Queue.push(30); ` `    ``Queue.push(40); ` `    ``Queue.push(50); ` `    ``Queue.push(60); ` `    ``Queue.push(70); ` `    ``Queue.push(80); ` `    ``Queue.push(90); ` `    ``Queue.push(100); ` ` `  `    ``reverseQueue(Queue); ` `    ``Print(Queue); ` `} `

## Java

 `// Java program to reverse a Queue ` `import` `java.util.LinkedList; ` `import` `java.util.Queue; ` `import` `java.util.Stack; ` ` `  `// Java program to reverse a queue ` `public` `class` `Queue_reverse { ` `     `  `    ``static` `Queue queue; ` ` `  `    ``// Utility function to print the queue ` `    ``static` `void` `Print() ` `    ``{ ` `        ``while` `(!queue.isEmpty()) { ` `            ``System.out.print( queue.peek() + ``", "``); ` `            ``queue.remove(); ` `        ``} ` `    ``} ` ` `  `    ``// Function to reverse the queue ` `    ``static` `void` `reversequeue() ` `    ``{ ` `        ``Stack stack = ``new` `Stack<>(); ` `        ``while` `(!queue.isEmpty()) { ` `            ``stack.add(queue.peek()); ` `            ``queue.remove(); ` `        ``} ` `        ``while` `(!stack.isEmpty()) { ` `            ``queue.add(stack.peek()); ` `            ``stack.pop(); ` `        ``} ` `    ``} ` ` `  `    ``// Driver code ` `    ``public` `static` `void` `main(String args[]) ` `    ``{ ` `        ``queue = ``new` `LinkedList(); ` `        ``queue.add(``10``); ` `        ``queue.add(``20``); ` `        ``queue.add(``30``); ` `        ``queue.add(``40``); ` `        ``queue.add(``50``); ` `        ``queue.add(``60``); ` `        ``queue.add(``70``); ` `        ``queue.add(``80``); ` `        ``queue.add(``90``); ` `        ``queue.add(``100``); ` ` `  `        ``reversequeue(); ` `        ``Print(); ` `    ``} ` `} ` `//This code is contributed by Sumit Ghosh `

## Python3

 `# Python3 program to reverse a queue  ` `from` `queue ``import` `Queue  ` ` `  `# Utility function to print the queue  ` `def` `Print``(queue): ` `    ``while` `(``not` `queue.empty()): ` `        ``print``(queue.queue[``0``], end ``=` `", "``)  ` `        ``queue.get() ` ` `  `# Function to reverse the queue  ` `def` `reversequeue(queue): ` `    ``Stack ``=` `[]  ` `    ``while` `(``not` `queue.empty()):  ` `        ``Stack.append(queue.queue[``0``])  ` `        ``queue.get() ` `    ``while` `(``len``(Stack) !``=` `0``):  ` `        ``queue.put(Stack[``-``1``])  ` `        ``Stack.pop() ` ` `  `# Driver code  ` `if` `__name__ ``=``=` `'__main__'``: ` `    ``queue ``=` `Queue() ` `    ``queue.put(``10``)  ` `    ``queue.put(``20``)  ` `    ``queue.put(``30``)  ` `    ``queue.put(``40``)  ` `    ``queue.put(``50``)  ` `    ``queue.put(``60``)  ` `    ``queue.put(``70``)  ` `    ``queue.put(``80``)  ` `    ``queue.put(``90``)  ` `    ``queue.put(``100``)  ` ` `  `    ``reversequeue(queue)  ` `    ``Print``(queue) ` ` `  `# This code is contributed by PranchalK `

## C#

 `// c# program to reverse a Queue  ` `using` `System; ` `using` `System.Collections.Generic; ` ` `  `public` `class` `GFG ` `{ ` ` `  `public` `static` `LinkedList<``int``> queue; ` ` `  `// Utility function to print the queue  ` `public` `static` `void` `Print() ` `{ ` `    ``while` `(queue.Count > 0) ` `    ``{ ` `        ``Console.Write(queue.First.Value + ``", "``); ` `        ``queue.RemoveFirst(); ` `    ``} ` `} ` ` `  `// Function to reverse the queue  ` `public` `static` `void` `reversequeue() ` `{ ` `    ``Stack<``int``> stack = ``new` `Stack<``int``>(); ` `    ``while` `(queue.Count > 0) ` `    ``{ ` `        ``stack.Push(queue.First.Value); ` `        ``queue.RemoveFirst(); ` `    ``} ` `    ``while` `(stack.Count > 0) ` `    ``{ ` `        ``queue.AddLast(stack.Peek()); ` `        ``stack.Pop(); ` `    ``} ` `} ` ` `  `// Driver code  ` `public` `static` `void` `Main(``string``[] args) ` `{ ` `    ``queue = ``new` `LinkedList<``int``>(); ` `    ``queue.AddLast(10); ` `    ``queue.AddLast(20); ` `    ``queue.AddLast(30); ` `    ``queue.AddLast(40); ` `    ``queue.AddLast(50); ` `    ``queue.AddLast(60); ` `    ``queue.AddLast(70); ` `    ``queue.AddLast(80); ` `    ``queue.AddLast(90); ` `    ``queue.AddLast(100); ` ` `  `    ``reversequeue(); ` `    ``Print(); ` `} ` `} ` ` `  `// This code is contributed by Shrikant13 `

Output:

```100, 90, 80, 70, 60, 50, 40, 30, 20, 10
```

Complexity Analysis:

• Time Complexity: O(n).
As we need to insert all the elements in the stack and later to the queue.
• Auxiliary Space: O(N).
Use of stack to store values.

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