Given an integer k and a queue of integers, we need to reverse the order of the first k elements of the queue, leaving the other elements in the same relative order.
Only following standard operations are allowed on queue.
- enqueue(x) : Add an item x to rear of queue
- dequeue() : Remove an item from front of queue
- size(( : Returns number of elements in queue.
- front() : Finds front item.
Examples:
Input : Q = [10, 20, 30, 40, 50, 60,
70, 80, 90, 100]
k = 5
Output : Q = [50, 40, 30, 20, 10, 60,
70, 80, 90, 100]
Input : Q = [10, 20, 30, 40, 50, 60,
70, 80, 90, 100]
k = 4
Output : Q = [40, 30, 20, 10, 50, 60,
70, 80, 90, 100]
The idea is to use an auxiliary stack.
1) Create an empty stack.
2) One by one dequeue items from given queue and push the dequeued items to stack.
3) Enqueue the contents of stack at the back of the queue
4) Reverse the whole queue.
C++
// C++ program to reverse first k elements of a queue. #include <bits/stdc++.h> using namespace std; /* Function to reverse the first K elements of the Queue */void reverseQueueFirstKElements(int k, queue<int>& Queue) { if (Queue.empty() == true || k > Queue.size()) return; if (k <= 0) return; stack<int> Stack; /* Push the first K elements into a Stack*/ for (int i = 0; i < k; i++) { Stack.push(Queue.front()); Queue.pop(); } /* Enqueue the contents of stack at the back of the queue*/ while (!Stack.empty()) { Queue.push(Stack.top()); Stack.pop(); } /* Remove the remaining elements and enqueue them at the end of the Queue*/ for (int i = 0; i < Queue.size() - k; i++) { Queue.push(Queue.front()); Queue.pop(); } } /* Utility Function to print the Queue */void Print(queue<int>& Queue) { while (!Queue.empty()) { cout << Queue.front() << " "; Queue.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); int k = 5; reverseQueueFirstKElements(k, Queue); Print(Queue); } |
Java
// Java program to reverse first k elements // of a queue. import java.util.LinkedList; import java.util.Queue; import java.util.Stack; public class Reverse_k_element_queue { static Queue<Integer> queue; // Function to reverse the first K elements // of the Queue static void reverseQueueFirstKElements(int k) { if (queue.isEmpty() == true || k > queue.size()) return; if (k <= 0) return; Stack<Integer> stack = new Stack<Integer>(); // Push the first K elements into a Stack for (int i = 0; i < k; i++) { stack.push(queue.peek()); queue.remove(); } // Enqueue the contents of stack at the back // of the queue while (!stack.empty()) { queue.add(stack.peek()); stack.pop(); } // Remove the remaining elements and enqueue // them at the end of the Queue for (int i = 0; i < queue.size() - k; i++) { queue.add(queue.peek()); queue.remove(); } } // Utility Function to print the Queue static void Print() { while (!queue.isEmpty()) { System.out.print(queue.peek() + " "); queue.remove(); } } // Driver code public static void main(String args[]) { queue = new LinkedList<Integer>(); 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); int k = 5; reverseQueueFirstKElements(k); Print(); } } // This code is contributed by Sumit Ghosh |
Python3
# Python3 program to reverse first k
# elements of a queue.
from queue import Queue
# Function to reverse the first K
# elements of the Queue
def reverseQueueFirstKElements(k,Queue):
if (Queue.empty() == True or
k > Queue.qsize()):
return
if (k <= 0):
return
Stack = []
# put the first K elements
# into a Stack
for i in range(k):
Stack.append(Queue.queue[0])
Queue.get()
# Enqueue the contents of stack
# at the back of the queue
while (len(Stack) != 0 ):
Queue.put(Stack[-1])
Stack.pop()
# Remove the remaining elements and
# enqueue them at the end of the Queue
for i in range(Queue.qsize() - k):
Queue.put(Queue.queue[0])
Queue.get()
# Utility Function to print the Queue
def Print(Queue):
while (not Queue.empty()):
print(Queue.queue[0],end=" ")
Queue.get()
# 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)
k = 5
reverseQueueFirstKElements(k, Queue)
Print(Queue)
# This code is contributed by PranchalK
[tabby title="C#"]
// C# program to reverse first k elements // of a queue. using System; using System.Collections.Generic; class GFG { public static LinkedList<int> queue; // Function to reverse the first K // elements of the Queue public static void reverseQueueFirstKElements(int k) { if (queue.Count == 0 || k > queue.Count) { return; } if (k <= 0) { return; } Stack<int> stack = new Stack<int>(); // Push the first K elements into a Stack for (int i = 0; i < k; i++) { stack.Push(queue.First.Value); queue.RemoveFirst(); } // Enqueue the contents of stack at // the back of the queue while (stack.Count > 0) { queue.AddLast(stack.Peek()); stack.Pop(); } // Remove the remaining elements and // enqueue them at the end of the Queue for (int i = 0; i < queue.Count - k; i++) { queue.AddLast(queue.First.Value); queue.RemoveFirst(); } } // Utility Function to print the Queue public static void Print() { while (queue.Count > 0) { Console.Write(queue.First.Value + " "); queue.RemoveFirst(); } } // 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); int k = 5; reverseQueueFirstKElements(k); Print(); } } // This code is contributed by Shrikant13 |
Output:
50 40 30 20 10 60 70 80 90 100
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This article is attributed to GeeksforGeeks.org
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