Open In App
Related Articles

Merging and Sorting Two Unsorted Stacks

Improve Article
Improve
Save Article
Save
Like Article
Like

Given 2 input stacks with elements in an unsorted manner. Problem is to merge them into a new final stack, such that the elements become arranged in a sorted manner. 

snip1

snip-3

Examples: 

Input :  s1 : 9 4 2 1
         s2: 8 17 3 10
Output : final stack: 1 2 3 4 8 9 10 17

Input :  s1 : 5 7 2 6 4       
         s2 : 12 9 3 
Output : final stack: 2 3 4 5 6 7 9 12

Create an empty stack to store result. We first insert elements of both stacks into the result. Then we sort the result stack

Implementation:

C++




// C++ program to merge to unsorted stacks
// into a third stack in sorted way.
#include <bits/stdc++.h>
using namespace std;
 
// Sorts input stack and returns sorted stack.
stack<int> sortStack(stack<int>& input)
{
    stack<int> tmpStack;
 
    while (!input.empty()) {
        // pop out the first element
        int tmp = input.top();
        input.pop();
 
        // while temporary stack is not empty and top
        // of stack is greater than temp
        while (!tmpStack.empty() && tmpStack.top() > tmp) {
 
            // pop from temporary stack and push
            // it to the input stack
            input.push(tmpStack.top());
            tmpStack.pop();
        }
 
        // push temp in temporary of stack
        tmpStack.push(tmp);
    }
 
    return tmpStack;
}
 
stack<int> sortedMerge(stack<int>& s1, stack<int>& s2)
{
    // Push contents of both stacks in result
    stack<int> res;
    while (!s1.empty()) {
        res.push(s1.top());
        s1.pop();
    }
    while (!s2.empty()) {
        res.push(s2.top());
        s2.pop();
    }
 
    // Sort the result stack.
    return sortStack(res);
}
 
// main function
int main()
{
    stack<int> s1, s2;
    s1.push(34);
    s1.push(3);
    s1.push(31);
 
    s2.push(1);
    s2.push(12);
    s2.push(23);
 
    // This is the temporary stack
    stack<int> tmpStack = sortedMerge(s1, s2);
    cout << "Sorted and merged stack :\n";
 
    while (!tmpStack.empty()) {
        cout << tmpStack.top() << " ";
        tmpStack.pop();
    }
}


Java




// Java program to merge to unsorted stacks
// into a third stack in sorted way.
import java.io.*;
import java.util.*;
 
public class GFG {
     
    // This is the temporary stack
    static Stack<Integer> res = new Stack<Integer>();
    static Stack<Integer> tmpStack = new Stack<Integer>();
     
    // Sorts input stack and returns
    // sorted stack.
    static void sortStack(Stack<Integer> input)
    {
        while (input.size() != 0)
        {
            // pop out the first element
            int tmp = input.peek();
            input.pop();
     
            // while temporary stack is not empty and
            // top of stack is greater than temp
            while (tmpStack.size() != 0 &&
                            tmpStack.peek() > tmp)
            {
     
                // pop from temporary stack and push
                // it to the input stack
                input.push(tmpStack.peek());
                tmpStack.pop();
            }
     
            // push temp in temporary of stack
            tmpStack.push(tmp);
        }
    }
     
    static void sortedMerge(Stack<Integer> s1,
                                Stack<Integer> s2)
    {
        // Push contents of both stacks in result
        while (s1.size() != 0) {
            res.push(s1.peek());
            s1.pop();
        }
         
        while (s2.size() != 0) {
            res.push(s2.peek());
            s2.pop();
        }
     
        // Sort the result stack.
        sortStack(res);
    }
     
    // main function
    public static void main(String args[])
    {
        Stack<Integer> s1 = new Stack<Integer>();
        Stack<Integer> s2 = new Stack<Integer>();
        s1.push(34);
        s1.push(3);
        s1.push(31);
     
        s2.push(1);
        s2.push(12);
        s2.push(23);
     
        sortedMerge(s1, s2);
        System.out.println("Sorted and merged stack :");
     
        while (tmpStack.size() != 0) {
            System.out.print(tmpStack.peek() + " ");
            tmpStack.pop();
        }
    }
}
 
// This code is contributed by Manish Shaw
// (manishshaw1)


Python3




# Python3 program to merge to unsorted stacks
# into a third stack in sorted way.
 
# Sorts input stack and returns
# sorted stack.
def sortStack(Input):
    tmpStack = []
   
    while len(Input) != 0:
        # pop out the first element
        tmp = Input[-1]
        Input.pop()
   
        # while temporary stack is not empty and
        # top of stack is greater than temp
        while len(tmpStack) != 0 and tmpStack[-1] > tmp:
            # pop from temporary stack and push
            # it to the input stack
            Input.append(tmpStack[-1])
            tmpStack.pop()
        # push temp in temporary of stack
        tmpStack.append(tmp)
    return tmpStack
   
def sortedMerge(s1, s2):
    # Push contents of both stacks in result
    res = []
    while len(s1) !=0 :
        res.append(s1[-1])
        s1.pop()
    while len(s2) !=0 :
        res.append(s2[-1])
        s2.pop()
   
    # Sort the result stack.
    return sortStack(res)
 
s1 = []
s2 = []
s1.append(34)
s1.append(3)
s1.append(31)
 
s2.append(1)
s2.append(12)
s2.append(23)
 
# This is the temporary stack
tmpStack = []
tmpStack = sortedMerge(s1, s2)
print("Sorted and merged stack :")
 
while len(tmpStack) != 0 :
    print(tmpStack[-1], end = " ")
    tmpStack.pop()
     
    # This code is contributed by decode2207.


C#




// C# program to merge to unsorted stacks
// into a third stack in sorted way.
using System;
using System.Collections.Generic;
 
class GFG {
     
    // Sorts input stack and returns
    // sorted stack.
    static Stack<int> sortStack(ref Stack<int> input)
    {
        Stack<int> tmpStack = new Stack<int>();
     
        while (input.Count != 0)
        {
            // pop out the first element
            int tmp = input.Peek();
            input.Pop();
     
            // while temporary stack is not empty and
            // top of stack is greater than temp
            while (tmpStack.Count != 0 &&
                              tmpStack.Peek() > tmp)
            {
     
                // pop from temporary stack and push
                // it to the input stack
                input.Push(tmpStack.Peek());
                tmpStack.Pop();
            }
     
            // push temp in temporary of stack
            tmpStack.Push(tmp);
        }
     
        return tmpStack;
    }
     
    static Stack<int> sortedMerge(ref Stack<int> s1,
                                  ref Stack<int> s2)
    {
        // Push contents of both stacks in result
        Stack<int> res = new Stack<int>();
        while (s1.Count!=0) {
            res.Push(s1.Peek());
            s1.Pop();
        }
        while (s2.Count!=0) {
            res.Push(s2.Peek());
            s2.Pop();
        }
     
        // Sort the result stack.
        return sortStack(ref res);
    }
     
    // main function
    static void Main()
    {
        Stack<int> s1 = new Stack<int>();
        Stack<int> s2 = new Stack<int>();
        s1.Push(34);
        s1.Push(3);
        s1.Push(31);
     
        s2.Push(1);
        s2.Push(12);
        s2.Push(23);
     
        // This is the temporary stack
        Stack<int> tmpStack = new Stack<int>();
        tmpStack = sortedMerge(ref s1,ref s2);
        Console.Write("Sorted and merged stack :\n");
     
        while (tmpStack.Count!=0) {
            Console.Write(tmpStack.Peek() + " ");
            tmpStack.Pop();
        }
    }
}
 
// This code is contributed by Manish Shaw
// (manishshaw1)


Javascript




<script>
    // Javascript program to merge to unsorted stacks
    // into a third stack in sorted way.
     
    // This is the temporary stack
    let res = [];
    let tmpStack = [];
      
    // Sorts input stack and returns
    // sorted stack.
    function sortStack(input)
    {
        while (input.length != 0)
        {
            // pop out the first element
            let tmp = input[input.length - 1];
            input.pop();
      
            // while temporary stack is not empty and
            // top of stack is greater than temp
            while (tmpStack.length != 0 &&
                            tmpStack[tmpStack.length - 1] > tmp)
            {
      
                // pop from temporary stack and push
                // it to the input stack
                input.push(tmpStack[tmpStack.length - 1]);
                tmpStack.pop();
            }
      
            // push temp in temporary of stack
            tmpStack.push(tmp);
        }
    }
      
    function sortedMerge(s1, s2)
    {
        // Push contents of both stacks in result
        while (s1.length != 0) {
            res.push(s1[s1.length - 1]);
            s1.pop();
        }
          
        while (s2.length != 0) {
            res.push(s2[s2.length - 1]);
            s2.pop();
        }
      
        // Sort the result stack.
        sortStack(res);
    }
     
    let s1 = [];
    let s2 = [];
    s1.push(34);
    s1.push(3);
    s1.push(31);
 
    s2.push(1);
    s2.push(12);
    s2.push(23);
 
    sortedMerge(s1, s2);
    document.write("Sorted and merged stack :" + "</br>");
 
    while (tmpStack.length != 0) {
      document.write(tmpStack[tmpStack.length - 1] + " ");
      tmpStack.pop();
    }
     
    // This code is contributed by divyesh072019.
</script>


Output

Sorted and merged stack :
34 31 23 12 3 1 

Complexity Analysis:

  • Time Complexity: O((n+m)2) where n and m are number of elements in both the stacks respectively.
  • Auxiliary Space: O(n+m) 

Feeling lost in the world of random DSA topics, wasting time without progress? It's time for a change! Join our DSA course, where we'll guide you on an exciting journey to master DSA efficiently and on schedule.
Ready to dive in? Explore our Free Demo Content and join our DSA course, trusted by over 100,000 geeks!

Last Updated : 17 Aug, 2022
Like Article
Save Article
Previous
Next
Similar Reads
Complete Tutorials