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Check if the elements of stack are pairwise sorted
  • Difficulty Level : Medium
  • Last Updated : 01 Jul, 2019

Given a stack of integers, write a function pairWiseSorted() that checks whether numbers in the stack are pairwise sorted or not.
The pairs must be increasing, and if the stack has an odd number of elements, the element at the top is left out of a pair. The function should retain the original stack content.

Only following standard operations are allowed on the stack.

  1. push(X): Enter a element X on top of stack.
  2. pop(): Removes top element of the stack.
  3. empty(): To check if stack is empty.

Examples:

Input: 4, 5, 6, 7, 8, 9
Output: Yes

Input: 4, 9, 2, 1, 10, 8
Output: No

Approach: The idea is to use another stack.

  • Create an auxiliary stack aux.
  • Transfer contents of given stack to aux.
  • Traverse aux. While traversing fetch top two elements and check if they are sorted or not.
  • After checking put these elements back to original stack.

Below is the implementation of above approach:

C++




// C++ program to check if successive
// pair of numbers in the stack are
// sorted or not
#include <bits/stdc++.h>
using namespace std;
  
// Function to check if elements are
// pairwise sorted in stack
bool pairWiseConsecutive(stack<int> s)
{
    // Transfer elements of s to aux.
    stack<int> aux;
    while (!s.empty()) {
        aux.push(s.top());
        s.pop();
    }
  
    // Traverse aux and see if
    // elements are pairwise
    // sorted or not. We also
    // need to make sure that original
    // content is retained.
    bool result = true;
    while (!aux.empty()) {
  
        // Fetch current top two
        // elements of aux and check
        // if they are sorted.
        int x = aux.top();
        aux.pop();
        int y = aux.top();
        aux.pop();
        if (x > y)
            result = false;
  
        // Push the elements to original
        // stack.
        s.push(x);
        s.push(y);
    }
  
    if (aux.size() == 1)
        s.push(aux.top());
  
    return result;
}
  
// Driver program
int main()
{
    stack<int> s;
    s.push(4);
    s.push(5);
    s.push(-3);
    s.push(-2);
    s.push(10);
    s.push(11);
    s.push(5);
    s.push(6);
    // s.push(20);
  
    if (pairWiseConsecutive(s))
        cout << "Yes" << endl;
    else
        cout << "No" << endl;
  
    cout << "Stack content (from top)"
            " after function call\n";
    while (s.empty() == false) {
        cout << s.top() << " ";
        s.pop();
    }
  
    return 0;
}


Java




// Java program to check if successive 
// pair of numbers in the stack are 
// sorted or not 
import java.util.Stack;
  
class GFG {
  
// Function to check if elements are 
// pairwise sorted in stack 
    static boolean pairWiseConsecutive(Stack<Integer> s) {
        // Transfer elements of s to aux. 
        Stack<Integer> aux = new Stack<>();
        while (!s.empty()) {
            aux.push(s.peek());
            s.pop();
        }
  
        // Traverse aux and see if 
        // elements are pairwise 
        // sorted or not. We also 
        // need to make sure that original 
        // content is retained. 
        boolean result = true;
        while (!aux.empty()) {
  
            // Fetch current top two 
            // elements of aux and check 
            // if they are sorted. 
            int x = aux.peek();
            aux.pop();
            int y = aux.peek();
            aux.pop();
            if (x > y) {
                result = false;
            }
  
            // Push the elements to original 
            // stack. 
            s.push(x);
            s.push(y);
        }
  
        if (aux.size() == 1) {
            s.push(aux.peek());
        }
  
        return result;
    }
  
// Driver program 
    public static void main(String[] args) {
  
        Stack<Integer> s = new Stack<>();
        s.push(4);
        s.push(5);
        s.push(-3);
        s.push(-2);
        s.push(10);
        s.push(11);
        s.push(5);
        s.push(6);
        // s.push(20); 
  
        if (pairWiseConsecutive(s)) {
            System.out.println("Yes");
        } else {
            System.out.println("No");
        }
  
        System.out.println("Stack content (from top)"
                + " after function call");
        while (s.empty() == false) {
            System.out.print(s.peek() + " ");
            s.pop();
        }
  
    }
  
}


Python 3




# Python program to check if successive
# pair of numbers in the stack are
# sorted or not
  
# using deque as stack
from collections import deque
  
# Function to check if elements are
# pairwise sorted in stack
def pairWiseConsecutive(s):
  
    # Transfer elements of s to aux.
    aux = deque()
    while len(s) > 0:
        aux.append(s.pop())
  
    # Traverse aux and see if
    # elements are pairwise
    # sorted or not. We also
    # need to make sure that original
    # content is retained.
    result = True
    while len(aux) != 0:
  
        # Fetch current top two
        # elements of aux and check
        # if they are sorted.
        x = aux.pop()
        y = aux.pop()
        if x > y:
            result = False
  
        # Push the elements to original
        # stack.
        s.append(x)
        s.append(y)
  
    if len(aux) == 1:
        s.append(aux.pop())
  
    return result
  
  
# Driver Code
if __name__ == "__main__":
    s = deque()
    s.append(4)
    s.append(5)
    s.append(-3)
    s.append(-2)
    s.append(10)
    s.append(11)
    s.append(5)
    s.append(6)
  
    if pairWiseConsecutive(s):
        print("Yes")
    else:
        print("No")
  
    print("Stack content (from top) after function call")
    while len(s) > 0:
        print(s.pop(), end=" ")
  
# This code is contributed by
# sanjeev2552


C#




// C# program to check if successive 
// pair of numbers in the stack are 
using System;
using System.Collections.Generic;
public class GFG{
  
// Function to check if elements are 
// pairwise sorted in stack 
    static bool pairWiseConsecutive(Stack<int> s) {
        // Transfer elements of s to aux. 
        Stack<int> aux = new Stack<int>();
        while (!(s.Count==0)) {
            aux.Push(s.Peek());
            s.Pop();
        }
  
        // Traverse aux and see if 
        // elements are pairwise 
        // sorted or not. We also 
        // need to make sure that original 
        // content is retained. 
        bool result = true;
        while (!(aux.Count==0)) {
  
            // Fetch current top two 
            // elements of aux and check 
            // if they are sorted. 
            int x = aux.Peek();
            aux.Pop();
            int y = aux.Peek();
            aux.Pop();
            if (x > y) {
                result = false;
            }
  
            // Push the elements to original 
            // stack. 
            s.Push(x);
            s.Push(y);
        }
  
        if (aux.Count == 1) {
            s.Push(aux.Peek());
        }
  
        return result;
    }
  
// Driver program 
    public static void Main() {
  
        Stack<int> s = new Stack<int>();
        s.Push(4);
        s.Push(5);
        s.Push(-3);
        s.Push(-2);
        s.Push(10);
        s.Push(11);
        s.Push(5);
        s.Push(6);
        // s.push(20); 
  
        if (pairWiseConsecutive(s)) {
            Console.WriteLine("Yes");
        } else {
            Console.WriteLine("No");
        }
  
        Console.WriteLine("Stack content (from top)"
                + " after function call");
        while (!(s.Count == 0)) {
            Console.Write(s.Peek() + " ");
            s.Pop();
        }
  
    }
  
}


Output:

Yes
Stack content (from top) after function call
6 5 11 10 -2 -3 5 4

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