Reversing a queue using recursion
Given a queue, write a recursive function to reverse it.
Standard operations allowed :
enqueue(x) : Add an item x to rear of queue.
dequeue() : Remove an item from front of queue.
empty() : Checks if a queue is empty or not.
Examples :
Input : Q = [5, 24, 9, 6, 8, 4, 1, 8, 3, 6] Output : Q = [6, 3, 8, 1, 4, 8, 6, 9, 24, 5] Explanation : Output queue is the reverse of the input queue. Input : Q = [8, 7, 2, 5, 1] Output : Q = [1, 5, 2, 7, 8]
Recursive Algorithm :
- The pop element from the queue if the queue has elements otherwise return empty queue.
- Call reverseQueue function for the remaining queue.
- Push the popped element in the resultant reversed queue.
Pseudo Code :
queue reverseFunction(queue)
{
if (queue is empty)
return queue;
else {
data = queue.front()
queue.pop()
queue = reverseFunction(queue);
q.push(data);
return queue;
}
}
C++
// C++ code for reversing a queue #include <bits/stdc++.h> using namespace std; // Utility function to print the queue void printQueue(queue<long long int> Queue) { while (!Queue.empty()) { cout << Queue.front() << " "; Queue.pop(); } } // Recursive function to reverse the queue void reverseQueue(queue<long long int>& q) { // Base case if (q.empty()) return; // Dequeue current item (from front) long long int data = q.front(); q.pop(); // Reverse remaining queue reverseQueue(q); // Enqueue current item (to rear) q.push(data); } // Driver code int main() { queue<long long int> Queue; Queue.push(56); Queue.push(27); Queue.push(30); Queue.push(45); Queue.push(85); Queue.push(92); Queue.push(58); Queue.push(80); Queue.push(90); Queue.push(100); reverseQueue(Queue); printQueue(Queue); } |
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Java
// Java program to reverse a Queue by recursion import java.util.LinkedList; import java.util.Queue; import java.util.Stack; // Java program to reverse a queue recursively public class Queue_reverse { static Queue<Integer> queue; // Utility function to print the queue static void Print() { while (!queue.isEmpty()) { System.out.print(queue.peek() + " "); queue.remove(); } } // Recurrsive function to reverse the queue static Queue<Integer> reverseQueue(Queue<Integer> q) { // Base case if (q.isEmpty()) return q; // Dequeue current item (from front) int data = q.peek(); q.remove(); // Reverse remaining queue q = reverseQueue(q); // Enqueue current item (to rear) q.add(data); return q; } // Driver code public static void main(String args[]) { queue = new LinkedList<Integer>(); queue.add(56); queue.add(27); queue.add(30); queue.add(45); queue.add(85); queue.add(92); queue.add(58); queue.add(80); queue.add(90); queue.add(100); queue = reverseQueue(queue); Print(); } } |
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Python3
# Queue Class class Queue: def __init__(self): self.items = [] def isEmpty(self): return self.items == [] def add(self, item): self.items.append(item) def pop(self): return self.items.pop(0) def front(self): return self.items[0] def printQueue(self): for i in self.items: print(i, end =" ") print("") # Recursive Function to reverse the queue def reverseQueue(q): # Base case if (q.isEmpty()): return # Dequeue current item (from front) data = q.front(); q.pop(); # Reverse remaining queue reverseQueue(q) # Enqueue current item (to rear) q.add(data) # Driver Code q = Queue() q.add(56) q.add(27) q.add(30) q.add(45) q.add(85) q.add(92) q.add(58) q.add(80) q.add(90) q.add(100) reverseQueue(q) q.printQueue() |
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C#
// C# code for reversing a queue using System; using System.Collections.Generic; class GFG { // Utility function // to print the queue static void printQueue(Queue<long> queue) { while (queue.Count != 0) { Console.Write(queue.Peek() + " "); queue.Dequeue(); } } // Recursive function // to reverse the queue static void reverseQueue(ref Queue<long> q) { // Base case if (q.Count == 0) return; // Dequeue current // item (from front) long data = q.Peek(); q.Dequeue(); // Reverse remaining queue reverseQueue(ref q); // Enqueue current // item (to rear) q.Enqueue(data); } // Driver code static void Main() { Queue<long> queue = new Queue<long>(); queue.Enqueue(56); queue.Enqueue(27); queue.Enqueue(30); queue.Enqueue(45); queue.Enqueue(85); queue.Enqueue(92); queue.Enqueue(58); queue.Enqueue(80); queue.Enqueue(90); queue.Enqueue(100); reverseQueue(ref queue); printQueue(queue); } } // This code is contributed by // Manish Shaw(manishshaw1) |
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Output:
100 90 80 58 92 85 45 30 27 56
Time Complexity : O(n).
Video:
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