Stable sort for descending order

Difficulty Level : Basic
Last Updated : 15 Jul, 2019

Given an array of n integers, we have to reverse sort the array elements such the equal keys are stable after sorting.
Examples:

Input : arr[] = {4, 2, 3, 2, 4}
Output : 4, 4, 3, 2, 2

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Prerequisite : Stability in sorting algorithms

Method 1 (Writing our own sorting function : Bubble Sort)

We know sorting algorithms like Bubble Sort, Insertion Sort, Merge Sort, Count Sort are stable. We implement here Bubble Sort.

Explanation
First Pass
(4′, 2′, 3, 2″, 4″) -> (2′, 4′, 3, 4″, 2″) Here algorithm compares last two element and
swaps since 2″ < 4".
(2′, 4′, 3, 4″, 2″) -> (2′, 4′, 4″, 3, 2″) swap since 3 < 4"
(2′, 4′, 4″, 3, 2″) -> (2′, 4′, 4″, 3, 2″)
(2′, 4′, 4″, 3, 2″) -> (4′, 2′, 4″, 3, 2″) swap since 2′ < 4'.

Second Pass:
(4′, 2′, 4″, 3, 2″) -> (4′, 2′, 4″, 3, 2″)
(4′, 2′, 4″, 3, 2″) -> (4′, 2′, 4″, 3, 2″)
(4′, 2′, 4″, 3, 2″) -> (4′, 4″, 2′, 3, 2″) swap since 2′ (4′, 4″, 2′, 3, 2″)

Third Pass:
(4′, 4″, 2′, 3, 2″) -> (4′, 4″, 2′, 3, 2″)
(4′, 4″, 2′, 3, 2″) -> (4′, 4″, 3, 2′, 2″) swap since 2′<3
Now, the array is in sorted order and same elements are in same order as they were in the original array.

C++

// Bubble sort implementation to sort 
// elements in descending order. 
#include <iostream> 
#include <vector> 
using namespace std; 
  
void print(vector<int> a, int n) 
{ 
    for (int i = 0; i <= n; i++)  
        cout << a[i] << " ";     
    cout << endl; 
} 
  
// Sorts a[] in descending order using 
// bubble sort. 
void sort(vector<int> a, int n) 
{ 
    for (int i = n; i >= 0; i--)  
        for (int j = n; j > n - i; j--)  
            if (a[j] > a[j - 1])  
                swap(a[j], a[j-1]); 
    print(a, n); 
} 
  
// Driver code 
int main() 
{ 
    int n = 7; 
    vector<int> a; 
    a.push_back(2); 
    a.push_back(4); 
    a.push_back(3); 
    a.push_back(2); 
    a.push_back(4); 
    a.push_back(5); 
    a.push_back(3); 
    sort(a, n - 1); 
    return 0; 
} 

Java

// Bubble sort implementation  
// to sort elements in  
// descending order. 
import java.io.*; 
import java.util.*; 
  
class GFG 
{  
static void print(ArrayList<Integer> a,  
                                 int n) 
{ 
    for (int i = 0; i <= n; i++)  
        System.out.print(a.get(i) + " ");  
    System.out.println(); 
} 
  
// Sorts a[] in descending  
// order using bubble sort. 
static void sort(ArrayList<Integer> a, 
                                  int n) 
{ 
    for (int i = n;  
            i >= 0; i--)  
        for (int j = n;  
                j > n - i; j--)  
            if (a.get(j) > a.get(j - 1))  
            { 
                int tempswap = a.get(j); 
                a.remove(j); 
                a.add(j, a.get(j - 1));  
                a.remove(j - 1); 
                a.add(j - 1, tempswap);  
            } 
    print(a, n); 
} 
  
// Driver code 
public static void main(String[] args) 
{ 
    int n = 6; 
    ArrayList<Integer> a = new ArrayList<Integer>(); 
    a.add(2); 
    a.add(4); 
    a.add(3); 
    a.add(2); 
    a.add(4); 
    a.add(5); 
    a.add(3); 
    sort(a, n); 
} 
} 
  
// This code is contributed by  
// Manish Shaw(manishshaw1) 

Python3

# Bubble sort implementation to sort 
# elements in descending order. 
  
def print1(a, n): 
  
    for i in range(0,n+1): 
        print(a[i],end=" ")  
    print("") 
  
  
# Sorts a[] in descending order using 
# bubble sort. 
def sort(a, n): 
  
    for i in range(n,0,-1):  
        for j in range(n, n - i,-1):  
            if (a[j] > a[j - 1]):  
                a[j], a[j-1]=a[j-1], a[j] 
    print1(a,n) 
  
  
# Driver code 
n = 7
a = [2,4,3,2,4,5,3] 
  
sort(a, n-1) 
  
# This code is contributed 
# by Smitha Dinesh Semwal 

C#

// Bubble sort implementation  
// to sort elements in  
// descending order. 
using System; 
using System.Collections.Generic; 
  
class GFG 
{  
static void print(List<int> a,  
                       int n) 
{ 
    for (int i = 0; i <= n; i++)  
        Console.Write(a[i] + " ");  
    Console.WriteLine(); 
} 
  
// Sorts a[] in descending  
// order using bubble sort. 
static void sort(List<int> a, 
                      int n) 
{ 
    for (int i = n;  
             i >= 0; i--)  
        for (int j = n;  
                 j > n - i; j--)  
            if (a[j] > a[j - 1])  
            { 
                int tempswap = a[j]; 
                a[j] = a[j - 1];  
                a[j - 1] = tempswap;  
            } 
    print(a, n); 
} 
  
// Driver code 
static void Main() 
{ 
    int n = 6; 
    List<int> a = new List<int>(); 
    a.Add(2); 
    a.Add(4); 
    a.Add(3); 
    a.Add(2); 
    a.Add(4); 
    a.Add(5); 
    a.Add(3); 
    sort(a, n); 
} 
} 
  
// This code is contributed by  
// Manish Shaw(manishshaw1) 

PHP

<?php 
// Bubble sort implementation  
// to sort elements in 
// descending order. 
  
function swap(&$x, &$y) 
{ 
    $x ^= $y ^= $x ^= $y; 
} 
  
function print1($a, $n) 
{ 
    for ($i = 0; $i <= $n; $i++)  
        echo ($a[$i] . " "); 
    echo ("\n"); 
} 
  
// Sorts a[] in descending  
// order using bubble sort. 
function sort1($a, $n) 
{ 
    for ($i = $n; 
         $i >= 0; $i--) 
    { 
        for ($j = $n;  
             $j > $n - $i; $j--)  
        { 
            if ($a[$j] > $a[$j - 1])  
                swap($a[$j],  
                     $a[$j - 1]); 
        } 
    } 
    print1($a, $n); 
} 
  
// Driver code 
$n = 6; 
$a = array(); 
array_push($a, 2); 
array_push($a, 4); 
array_push($a, 3); 
array_push($a, 2); 
array_push($a, 4); 
array_push($a, 5); 
array_push($a, 3); 
sort1($a, $n); 
  
// This code is contributed by  
// Manish Shaw(manishshaw1) 
?> 

Output:

5 4 4 3 3 2 2

Method 2 (Using library function)
We can use stable_sort to sort elements in stable manner.

C++

// C++ program to demonstrate descending order 
// stable sort using greater<>(). 
#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
    int arr[] = { 1, 5, 8, 9, 6, 7, 3, 4, 2, 0 }; 
    int n = sizeof(arr) / sizeof(arr[0]); 
  
    stable_sort(arr, arr + n, greater<int>()); 
  
    cout << "Array after sorting : \n"; 
    for (int i = 0; i < n; ++i) 
        cout << arr[i] << " "; 
  
    return 0; 
} 

Java

// Java program to demonstrate descending order  
// stable sort using greater<>(). 
import java.util.*; 
  
class GFG  
{ 
    static void reverse(int a[])  
    {  
        int i, k, n = a.length;  
        int t; 
        for (i = 0; i < n / 2; i++) 
        {  
            t = a[i];  
            a[i] = a[n - i - 1];  
            a[n - i - 1] = t;  
        }  
    } 
      
    // Driver code  
    public static void main(String[] args) 
    { 
        int arr[] = {1, 5, 8, 9, 6, 7, 3, 4, 2, 0}; 
        int n = arr.length; 
  
        Arrays.sort(arr); 
        reverse(arr); 
  
        System.out.println("Array after sorting : \n"); 
        for (int i = 0; i < n; ++i) 
        { 
            System.out.print(arr[i] + " "); 
        } 
    } 
}  
  
// This code has been contributed by 29AjayKumar 

Python 3

# Python 3 program to demonstrate  
# descending order 
  
if __name__ == "__main__": 
      
    arr = [ 1, 5, 8, 9, 6, 
            7, 3, 4, 2, 0 ] 
    n = len(arr) 
  
    arr.sort(reverse = True) 
  
    print("Array after sorting : ") 
    for i in range(n): 
        print(arr[i], end = " ") 
  
# This code is contributed by ita_c 

C#

// C# program to demonstrate descending order  
using System; 
      
class GFG  
{ 
    static void reverse(int []a)  
    {  
        int i, k, n = a.Length;  
        int t; 
        for (i = 0; i < n / 2; i++) 
        {  
            t = a[i];  
            a[i] = a[n - i - 1];  
            a[n - i - 1] = t;  
        }  
    } 
      
    // Driver code  
    public static void Main(String[] args) 
    { 
        int []arr = {1, 5, 8, 9, 6, 7, 3, 4, 2, 0}; 
        int n = arr.Length; 
  
        Array.Sort(arr); 
        reverse(arr); 
  
        Console.WriteLine("Array after sorting : \n"); 
        for (int i = 0; i < n; ++i) 
        { 
            Console.Write(arr[i] + " "); 
        } 
    } 
}  
  
// This code is contributed by 29AjayKumar 

Output:

Array after sorting : 
9 8 7 6 5 4 3 2 1 0

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Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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Python 3

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