Sort a nearly sorted array using STL

Given an array of n elements, where each element is at most k away from its target position, devise an algorithm that sorts in O(n log k) time. For example, let us consider k is 2, an element at index 7 in the sorted array, can be at indexes 5, 6, 7, 8, 9 in the given array. It may be assumed that k < n.

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

Input : arr[] = {10, 9, 8, 7, 4, 70, 60, 50}
         k = 4
Output : arr[] = {4, 7, 8, 9, 10, 50, 60, 70}

A simple solution is to sort the array using any standard sorting algorithm. The time complexity of this solution is O(n Log n)

A better solution is to use priority queue (or heap data structure).

1) Build a priority queue pq of first (k+1) elements.
2) Initialize index = 0 (For result array).
3) Do following for elements from k+1 to n-1.
    a) Pop an item from pq and put it at index, increment index.
    b) Push arr[i] to pq.
4) While pq is not empty 
      Pop an item from pq and put it at index, increment index.

We have discussed a simple implementation in Sort a nearly sorted (or K sorted) array. In this post, an STL based implementation is done.

// A STL based C++ program to sort a nearly sorted array.
#include <bits/stdc++.h>
using namespace std;

// Given an array of size n, where every element
// is k away from its target position, sorts the
// array in O(nLogk) time.
int sortK(int arr[], int n, int k)
{
    // Insert first k+1 items in a priority queue (or min heap)
    //(A O(k) operation)
    priority_queue<int, vector<int>, greater<int> > pq(arr, arr + k + 1);

    // i is index for remaining elements in arr[] and index
    // is target index of for current minimum element in
    // Min Heapm 'hp'.
    int index = 0;
    for (int i = k + 1; i < n; i++) {
        arr[index++] = pq.top();
        pq.pop();
        pq.push(arr[i]);
    }

    while (pq.empty() == false) {
        arr[index++] = pq.top();
        pq.pop();
    }
}

// A utility function to print array elements
void printArray(int arr[], int size)
{
    for (int i = 0; i < size; i++)
        cout << arr[i] << " ";
    cout << endl;
}

// Driver program to test above functions
int main()
{
    int k = 3;
    int arr[] = { 2, 6, 3, 12, 56, 8 };
    int n = sizeof(arr) / sizeof(arr[0]);
    sortK(arr, n, k);

    cout << "Following is sorted arrayn";
    printArray(arr, n);

    return 0;
}
Output:

Following is sorted arrayn2 3 6 8 12 56

Time Complexity : O(n Log k)
Auxiliary Space : O(k)



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