Check if a queue can be sorted into another queue using a stack

Given a Queue consisting of first n natural numbers (in random order). The task is to check whether the given Queue elements can be arranged in increasing order in another Queue using a stack. The operation allowed are:
1. Push and pop elements from the stack
2. Pop (Or enqueue) from the given Queue.
3. Push (Or Dequeue) in the another Queue.

Examples :

Input : Queue[] = { 5, 1, 2, 3, 4 }
Output : Yes
Pop the first element of the given Queue i.e 5.
Push 5 into the stack.
Now, pop all the elements of the given Queue and push them to
second Queue.
Now, pop element 5 in the stack and push it to the second Queue.
 
Input : Queue[] = { 5, 1, 2, 6, 3, 4 }
Output : No
Push 5 to stack.
Pop 1, 2 from given Queue and push it to another Queue.
Pop 6 from given Queue and push to stack.
Pop 3, 4 from given Queue and push to second Queue.
Now, from using any of above operation, we cannot push 5
into the second Queue because it is below the 6 in the stack.



Observe, second Queue (which will contain the sorted element) takes inputs (or enqueue elements) either from given Queue or Stack. So, next expected (which will initially be 1) element must be present as a front element of given Queue or top element of the Stack. So, simply simulate the process for the second Queue by initializing the expected element as 1. And check if we can get expected element from the front of the given Queue or from the top of the Stack. If we cannot take it from the either of them then pop the front element of given Queue and push it in the Stack.
Also, observe, that the stack must also be sorted at each instance i.e the element at the top of the stack must be smallest in the stack. For eg. let x > y, then x will always be expected before y. So, x cannot be pushed before y in the stack. Therefore, we cannot push element with the higher value on the top of the element having lesser value.

Algorithm:
1. Initialize the expected_element = 1
2. Check if either front element of given Queue or top element of the stack have expected_element
….a) If yes, increment expected_element by 1, repeat step 2.
….b) Else, pop front of Queue and push it to the stack. If the popped element is greater than top of the Stack, return “No”.

Below is the implementation of this approach:

C++

filter_none

edit
close

play_arrow

link
brightness_4
code

// CPP Program to check if a queue of first 
// n natural number can be sorted using a stack
#include <bits/stdc++.h>
using namespace std;
  
// Function to check if given queue element 
// can be sorted into another queue using a
// stack.
bool checkSorted(int n, queue<int>& q)
{
    stack<int> st;
    int expected = 1;
    int fnt;
  
    // while given Queue is not empty.
    while (!q.empty()) {
        fnt = q.front();
        q.pop();
  
        // if front element is the expected element
        if (fnt == expected)
            expected++;
  
        else {
            // if stack is empty, push the element
            if (st.empty()) {
                st.push(fnt);
            }
  
            // if top element is less than element which
            // need to be pushed, then return fasle.
            else if (!st.empty() && st.top() < fnt) {
                return false;
            }
  
            // else push into the stack.
            else
                st.push(fnt);
        }
  
        // while expected element are coming from
        // stack, pop them out.
        while (!st.empty() && st.top() == expected) {
            st.pop();
            expected++;
        }
    }
  
    // if the final expected element value is equal
    // to initial Queue size and the stack is empty.
    if (expected - 1 == n && st.empty())
        return true;
  
    return false;
}
  
// Driven Program
int main()
{
    queue<int> q;
    q.push(5);
    q.push(1);
    q.push(2);
    q.push(3);
    q.push(4);
  
    int n = q.size();
  
    (checkSorted(n, q) ? (cout << "Yes") :
                         (cout << "No"));
  
    return 0;
}

chevron_right


Java

filter_none

edit
close

play_arrow

link
brightness_4
code

// Java Program to check if a queue 
// of first n natural number can
// be sorted using a stack
import java.io.*;
import java.util.*;
  
class GFG
{
    static Queue<Integer> q = 
                    new LinkedList<Integer>();
      
    // Function to check if given 
    // queue element can be sorted 
    // into another queue using a stack.
    static boolean checkSorted(int n)
    {
        Stack<Integer> st = 
                    new Stack<Integer>();
        int expected = 1;
        int fnt;
      
        // while given Queue
        // is not empty.
        while (q.size() != 0
        {
            fnt = q.peek();
            q.poll();
      
            // if front element is 
            // the expected element
            if (fnt == expected)
                expected++;
      
            else
            {
                // if stack is empty, 
                // push the element
                if (st.size() == 0
                {
                    st.push(fnt);
                }
      
                // if top element is less than 
                // element which need to be 
                // pushed, then return fasle.
                else if (st.size() != 0 && 
                         st.peek() < fnt) 
                {
                    return false;
                }
      
                // else push into the stack.
                else
                    st.push(fnt);
            }
      
            // while expected element are
            // coming from stack, pop them out.
            while (st.size() != 0 && 
                   st.peek() == expected) 
            {
                st.pop();
                expected++;
            }
        }
          
        // if the final expected element 
        // value is equal to initial Queue
        // size and the stack is empty.
        if (expected - 1 == n && 
                st.size() == 0)
            return true;
      
        return false;
    }
      
    // Driver Code
    public static void main(String args[])
    {
        q.add(5);
        q.add(1);
        q.add(2);
        q.add(3);
        q.add(4);
      
        int n = q.size();
  
        if (checkSorted(n))
            System.out.print("Yes");
        else
            System.out.print("No");
    }
}
  
// This code is contributed by 
// Manish Shaw(manishshaw1)

chevron_right


Python3

filter_none

edit
close

play_arrow

link
brightness_4
code

# Python Program to check if a queue of first 
# n natural number can be sorted using a stack 
from queue import Queue 
  
# Function to check if given queue element 
# can be sorted into another queue using a 
# stack. 
def checkSorted(n, q):
    st = [] 
    expected = 1
    fnt = None
  
    # while given Queue is not empty. 
    while (not q.empty()): 
        fnt = q.queue[0
        q.get() 
  
        # if front element is the 
        # expected element 
        if (fnt == expected): 
            expected += 1
  
        else:
              
            # if stack is empty, put the element 
            if (len(st) == 0):
                st.append(fnt)
  
            # if top element is less than element which 
            # need to be puted, then return fasle. 
            elif (len(st) != 0 and st[-1] < fnt): 
                return False
  
            # else put into the stack. 
            else:
                st.append(fnt)
  
        # while expected element are coming 
        # from stack, pop them out. 
        while (len(st) != 0 and 
                   st[-1] == expected): 
            st.pop() 
            expected += 1
  
    # if the final expected element value is equal 
    # to initial Queue size and the stack is empty. 
    if (expected - 1 == n and len(st) == 0): 
        return True
  
    return False
  
# Driver Code
if __name__ == '__main__':
    q = Queue()
    q.put(5
    q.put(1
    q.put(2
    q.put(3
    q.put(4
  
    n = q.qsize() 
  
    if checkSorted(n, q):
        print("Yes")
    else:
        print("No")
  
# This code is contributed by PranchalK

chevron_right


C#

filter_none

edit
close

play_arrow

link
brightness_4
code

// C# Program to check if a queue 
// of first n natural number can
// be sorted using a stack
using System;
using System.Linq;
using System.Collections.Generic;
  
class GFG
{
    // Function to check if given 
    // queue element can be sorted 
    // into another queue using a stack.
    static bool checkSorted(int n, 
                            ref Queue<int> q)
    {
        Stack<int> st = new Stack<int>();
        int expected = 1;
        int fnt;
      
        // while given Queue
        // is not empty.
        while (q.Count != 0) 
        {
            fnt = q.Peek();
            q.Dequeue();
      
            // if front element is 
            // the expected element
            if (fnt == expected)
                expected++;
      
            else 
            {
                // if stack is empty, 
                // push the element
                if (st.Count != 0) 
                {
                    st.Push(fnt);
                }
      
                // if top element is less than 
                // element which need to be 
                // pushed, then return fasle.
                else if (st.Count != 0 && 
                         st.Peek() < fnt) 
                {
                    return false;
                }
      
                // else push into the stack.
                else
                    st.Push(fnt);
            }
      
            // while expected element are
            // coming from stack, pop them out.
            while (st.Count != 0 && 
                   st.Peek() == expected) 
            {
                st.Pop();
                expected++;
            }
        }
        // if the final expected element 
        // value is equal to initial Queue
        // size and the stack is empty.
        if (expected - 1 == n && 
                st.Count == 0)
            return true;
      
        return false;
    }
      
    // Driver Code
    static void Main()
    {
        Queue<int> q = new Queue<int>();
        q.Enqueue(5);
        q.Enqueue(1);
        q.Enqueue(2);
        q.Enqueue(3);
        q.Enqueue(4);
      
        int n = q.Count;
  
        if (checkSorted(n, ref q))
            Console.Write("Yes");
        else
            Console.Write("No");
    }
}
  
// This code is contributed by 
// Manish Shaw(manishshaw1)

chevron_right


Output :

Yes

 

Video Contributed by Parul Shandilya



My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.





Article Tags :
Practice Tags :


1


Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.