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Sort a nearly sorted (or K sorted) array | Set 2 (Gap method – Shell sort)

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  • Last Updated : 03 Sep, 2021
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Given an array, arr[] of N elements, where each element is at most K away from its target position, the task is to devise an algorithm that sorts in O(N*log(K)) time.

Examples:

Input: arr[] = {10, 9, 8, 7, 4, 70, 60, 50}, K = 4
Output: 4 7 8 9 10 50 60 70
Explanation:
Follow the steps below to sort the array:

  1. Start with Gap = K(i.e. 4)
    • 10 9 8 7 4 70 60 50, swap the elements at indices 0 and 4. Then the array modifies to {4, 9, 8, 7, 10, 70, 60, 50}.
      4 9 8 7 10 70 60 50, Do not swap the elements at indices 1 and 5.
      4 9 8 7 10 70 60 50, Do not swap the elements at indices 2 and 6.
      4 9 8 7 10 70 60 50, Do not swap the elements at indices 3 and 7.
  2. Gap = ceiling of 4/2 = 2
    • 4 9 8 7 10 70 60 50, Do not swap the elements at indices 0 and 2.
      4 9 8 7 10 70 60 50, swap the elements at indices 1 and 3. Then the array modifies to {4, 7, 8, 9, 10, 70, 60, 50}.
      4 7 8 9 10 70 60 50, Do not swap the elements at indices 2 and 4.
      4 7 8 9 10 70 60 50, Do not swap the elements at indices 3 and 5.
      4 7 8 9 10 70 60 50, Do not swap the elements at indices 4 and 6.
      4 7 8 9 10 70 60 50, swap the elements at indices 5 and 7. Then the array modifies to {4, 7, 8, 9, 10, 70, 60, 50}.
      4 7 8 9 10 50 60 70
  3. Gap = ceiling of 2/2 = 1
    • 4 7 8 9 10 50 60 70, Do not swap the elements at indices 0 and 1.
      4 7 8 9 10 50 60 70, Do not swap the elements at indices 1 and 2.
      4 7 8 9 10 50 60 70, Do not swap the elements at indices 2 and 3.
      4 7 8 9 10 50 60 70, Do not swap the elements at indices 3 and 4.
      4 7 8 9 10 50 60 70, Do not swap the elements at indices 4 and 5.
      4 7 8 9 10 50 60 70, Do not swap the elements at indices 5 and 6.
      4 7 8 9 10 50 60 70, Do not swap the elements at indices 6 and 7.

Input: arr[] = {6, 5, 3, 2, 8, 10, 9}, K = 3  
Output: 2 3 5 6 8 9 10

Approach: The given problem Sort a nearly sorted (or K sorted) array is already solved. Here the idea is to use shell sorting to sort the array. The idea used here is similar to the merging step of the In-Place Merge Sort. Follow the steps below to solve the problem:

  • Initialize a variable, say Gap with a value K to sort every Gapth element of every sublist.
  • Iterate until Gap is greater than 0 and perform the following steps:
    • Iterate over the range [0, N-Gap] using the variable i, and in each iteration, if arr[i] is greater than the arr[i+Gap], then swap the array elements.
    • Update the Gap as Gap = ceil(Gap/2).
  • Finally, after completing the above step print the elements of the array arr[].

C++




// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to find the nextGap
int nextGap(double k)
{
    if (k < 2)
        return 0;
    return ceil(k / 2);
}
 
// A utility function to print the array
void printArray(int arr[], int n)
{
    for (int i = 0; i < n; i++)
        cout << arr[i] << " ";
}
// Function to sort a K sorted array
void kSort(int arr[], int K, int n)
{
   
    // Iterate until gap is atleast
    // greater than 0
    for (int gap = K; gap > 0; gap = nextGap(gap)) {
 
        // Iterate over the range [0, N]
        for (int i = 0; i + gap < n; i++) {
 
            // If arr[i] is greater
            // than arr[i+gap]
            if (arr[i] > arr[i + gap]) {
 
                // Swap arr[i] and
                // arr[i+gap]
                swap(arr[i], arr[i + gap]);
            }
        }
    }
    printArray(arr, n);
}
 
// Driver Code
int main()
{
 
    // Input
    int arr[] = { 10, 9, 8, 7, 4, 70, 60, 50 };
    int K = 3;
    int n = sizeof(arr) / sizeof(arr[0]);
   
    // Function call
    kSort(arr, K, n);
    return 0;
}
 
// This code is contributed by lokesh potta.

Java




// Java program for the above approach
import java.util.Iterator;
import java.util.PriorityQueue;
 
class GFG {
 
    // Function to sort a K sorted array
    static void kSort(int[] arr, int K)
    {
        // Iterate until gap is atleast
        // greater than 0
        for (int gap = K; gap > 0; gap = nextGap(gap)) {
 
            // Iterate over the range [0, N]
            for (int i = 0; i + gap < arr.length; i++) {
 
                // If arr[i] is greater
                // than arr[i+gap]
                if (arr[i] > arr[i + gap]) {
 
                    // Swap arr[i] and
                    // arr[i+gap]
                    swap(arr, i, i + gap);
                }
            }
        }
        printArray(arr);
    }
 
    // Function to find the nextGap
    static int nextGap(double k)
    {
        if (k < 2)
            return 0;
        return (int)Math.ceil(k / 2);
    }
 
    // Function to swap two elements
    // of the array arr[]
    static void swap(int[] arr, int i, int j)
    {
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }
 
    // A utility function to print the array
    private static void printArray(int[] arr)
    {
        for (int i = 0; i < arr.length; i++)
            System.out.print(arr[i] + " ");
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        // Input
        int arr[] = { 10, 9, 8, 7, 4, 70, 60, 50 };
        int K = 3;
 
        // Function call
        kSort(arr, K);
    }
}

Python3




# Python3 program for the above approach
import math
 
# Function to find the nextGap
def nextGap(k):
     
    if (k < 2):
        return 0
         
    return math.ceil(k / 2)
 
# A utility function to print array
def printArray(arr, n):
     
    for i in range(n):
        print(arr[i], end = " ")
 
# Function to sort a K sorted array
def kSort(arr, K, n):
   
    # Iterate until gap is atleast
    # greater than 0
    gap = K
     
    while (gap > 0):
         
        # Iterate over the range [0, N]
        i = 0
        while (i + gap < n):
             
            # If arr[i] is greater
            # than arr[i+gap]
            if (arr[i] > arr[i + gap]):
                 
                # Swap arr[i] and
                # arr[i+gap]
                arr[i], arr[i + gap] = arr[i + gap], arr[i]
                 
            i += 1
         
        gap = nextGap(gap)
         
    printArray(arr, n)
 
# Driver Code
 
# Input
arr = [ 10, 9, 8, 7, 4, 70, 60, 50 ]
K = 3
n = len(arr)
   
# Function call
kSort(arr, K, n)
 
# This code is contributed by target_2

C#




// C# program for the above approach
using System;
 
class GFG {
 
    // Function to sort a K sorted array
    static void kSort(int[] arr, int K)
    {
        // Iterate until gap is atleast
        // greater than 0
        for (int gap = K; gap > 0; gap = nextGap(gap)) {
 
            // Iterate over the range [0, N]
            for (int i = 0; i + gap < arr.Length; i++) {
 
                // If arr[i] is greater
                // than arr[i+gap]
                if (arr[i] > arr[i + gap]) {
 
                    // Swap arr[i] and
                    // arr[i+gap]
                    swap(arr, i, i + gap);
                }
            }
        }
        printArray(arr);
    }
 
    // Function to find the nextGap
    static int nextGap(double k)
    {
        if (k < 2)
            return 0;
        return (int)Math.Ceiling(k / 2);
    }
 
    // Function to swap two elements
    // of the array arr[]
    static void swap(int[] arr, int i, int j)
    {
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }
 
    // A utility function to print the array
    private static void printArray(int[] arr)
    {
        for (int i = 0; i < arr.Length; i++)
            Console.Write(arr[i] + " ");
    }
 
    // Driver Code
    public static void Main(string[] args)
    {
        // Input
        int []arr = { 10, 9, 8, 7, 4, 70, 60, 50 };
        int K = 3;
 
        // Function call
        kSort(arr, K);
    }
}
 
// This code is contributed by ukasp.

Javascript




<script>
 
// Javascript program for the above approach
 
// Function to find the nextGap
function nextGap(k)
{
    if (k < 2)
        return 0;
         
    return Math.ceil(k / 2);
}
 
// A utility function to print the array
function printArray(arr, n)
{
    for(let i = 0; i < n; i++)
        document.write(arr[i] + " ");
}
 
// Function to sort a K sorted array
function kSort(arr, K, n)
{
     
    // Iterate until gap is atleast
    // greater than 0
    for(let gap = K; gap > 0; gap = nextGap(gap))
    {
         
        // Iterate over the range [0, N]
        for(let i = 0; i + gap < n; i++)
        {
             
            // If arr[i] is greater
            // than arr[i+gap]
            if (arr[i] > arr[i + gap])
            {
                 
                // Swap arr[i] and
                // arr[i+gap]
                let temp = arr[i];
                arr[i] = arr[i + gap];
                arr[i + gap] = temp;
            }
        }
    }
    printArray(arr, n);
}
 
// Driver Code
 
// Input
let arr = [ 10, 9, 8, 7, 4, 70, 60, 50 ];
let K = 3;
let n = arr.length;
 
// Function call
kSort(arr, K, n);
 
// This code is contributed by _saurabh_jaiswal
 
</script>

Output

4 7 8 9 10 50 60 70 

Time Complexity: O(N*log K)
Auxiliary Space: O(1)


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