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

13 Jan, 2022
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 Dequeue) from the given Queue. 3. Push (Or Enqueue) 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, the next expected (which will initially be 1) element must be present as a front element of a 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 the expected element from the front of the given Queue or from the top of the Stack. If we cannot take it from either of them then pop the front element of the 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 the 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 an element with a higher value on the top of the element having a 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++
Java
Python3
C#
Javascript
// 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 false.            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 Programint 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;}
// Java Program to check if a queue// of first n natural number can// be sorted using a stackimport 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 false.                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)
# Python Program to check if a queue of first# n natural number can be sorted using a stackfrom 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 false.            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 Codeif __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
// C# Program to check if a queue// of first n natural number can// be sorted using a stackusing 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 false.                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)
<script>    // Javascript Program to check if a queue    // of first n natural number can    // be sorted using a stack         let q = [];          // Function to check if given    // queue element can be sorted    // into another queue using a stack.    function checkSorted(n)    {        let st = [];        let expected = 1;        let fnt;              // while given Queue        // is not empty.        while (q.length != 0)        {            fnt = q[0];            q.shift();                  // if front element is            // the expected element            if (fnt == expected)                expected++;                  else            {                // if stack is empty,                // push the element                if (st.length == 0)                {                    st.push(fnt);                }                      // if top element is less than                // element which need to be                // pushed, then return false.                else if (st.length != 0 &&                         st[st.length - 1] < fnt)                {                    return false;                }                      // else push into the stack.                else                    st.push(fnt);            }                  // while expected element are            // coming from stack, pop them out.            while (st.length != 0 &&                   st[st.length - 1] == 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.length == 0)            return true;              return false;    }         q.push(5);    q.push(1);    q.push(2);    q.push(3);    q.push(4);     let n = q.length;     if (checkSorted(n))      document.write("Yes");    else      document.write("No");       // This code is contributed by suresh07.</script>
Yes
 
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