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Add elements in start to sort the array | Variation of Stalin Sort
  • Last Updated : 06 Oct, 2020

Stalin sort (also ‘dictator sort‘ and ‘trump sort‘) is a nonsensical ‘sorting’ algorithm in which each element that is not in the correct order is simply eliminated from the list. 
This sorting algorithm is a less destructive variation of Stalin sort, that will actually sort the list: In this case, the elements that are not in order are moved to the start of the list in the order in which they appeared in the original list. The process is then repeated until the list is sorted. 

Examples: 

Input: arr[] = {2, 1, 4, 3, 6, 5, 8, 7, 10, 9} 
Output: 1 2 3 4 5 6 7 8 9 10 
Explanation: 
Those elements which were moved to the start of the list are marked bold: 
[2, 1, 4, 3, 6, 5, 8, 7, 10, 9] 
[1, 3, 5, 7, 9 2, 4, 6, 8, 10] 
[2, 4, 6, 8, 1, 3, 5, 7, 9, 10] 
[1, 3, 5, 7, 2, 4, 6, 8, 9, 10] 
[2, 4, 6, 1, 3, 5, 7, 8, 9, 10] 
[1, 3, 5, 2, 4, 6, 7, 8, 9, 10] 
[2, 4, 1, 3, 5, 6, 7, 8, 9, 10] 
[1, 3, 2, 4, 5, 6, 7, 8, 9, 10] 
[2, 1, 3, 4, 5, 6, 7, 8, 9, 10] 
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Input: [9, 10, 7, 8, 5, 6, 3, 4, 1, 2] 
Output: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] 
Explanation: 
[9, 10, 7, 8, 5, 6, 3, 4, 1, 2] 
[7, 8, 5, 6, 3, 4, 1, 2, 9, 10] 
[5, 6, 3, 4, 1, 2, 7, 8, 9, 10] 
[3, 4, 1, 2, 5, 6, 7, 8, 9, 10] 
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10] 

Approach: The idea is to push the elements at the start of the array whenever it is less than the previous element. Repeat this step until there is no such element that is not in the correct order.



Below is the implementation of the above approach:

C++

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// C++ implementation to sort the
// array by using the variation
// of the Stalin sort
#include<bits/stdc++.h>
using namespace std;
 
// Function to sort the array
void variationStalinsort(vector<int> arr)
{
    int j = 0;
     
    while(true)
    {
        int moved = 0;
         
        for(int i = 0;
                i < (arr.size() - 1 - j); i++)
        {
            if (arr[i] > arr[i + 1])
            {
                vector<int>::iterator index;
                int temp;
                index = arr.begin() + i + 1;
                temp = arr[i + 1];
                arr.erase(index);
                arr.insert(arr.begin() + moved, temp);
                moved++;
            }
        }
         
        j++;
         
        if (moved == 0)
        {
            break;
        }
    }
    for(int i = 0; i < arr.size(); i++)
    {
        cout << arr[i] << ", ";
    }
}
 
// Driver Code
int main()
{
    vector<int> arr = { 2, 1, 4, 3, 6,
                        5, 8, 7, 10, 9 };
     
    // Function call
    variationStalinsort(arr);
}
 
// This code is contributed by avanitrachhadiya2155

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Python3

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# Python3 implementation to sort
# the array by using the variation
# of the Stalin sort
 
# Function to sort the array
def variationStalinsort(arr):
    j = 0
    while True:
        moved = 0
        for i in range(len(arr) - 1 - j):
            if arr[i] > arr[i + 1]:
                arr.insert(moved, arr.pop(i + 1))
                moved += 1
        j += 1
        if moved == 0:
            break
    return arr
 
# Driver Code
if __name__ == "__main__":
    arr = [2, 1, 4, 3, 6, 5, 8, 7, 10, 9]
     
    # Function Call
    print(variationStalinsort(arr))

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Output: 

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

 

Worst Time Complexity: O(N2
Best Time Complexity: O(N)
 

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