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Kth Smallest Element in a sorted array formed by reversing subarrays from a random index

  • Difficulty Level : Hard
  • Last Updated : 19 Jul, 2021

Given a sorted array arr[] of size N and an integer K, the task is to find Kth smallest element present in the array. The given array has been obtained by reversing subarrays {arr[0], arr[R]} and {arr[R + 1], arr[N – 1]} at some random index R. If the key is not present in the array, print -1.

Examples:

Input: arr[] = { 4, 3, 2, 1, 8, 7, 6, 5 }, K = 2
Output: 2
Explanation: Sorted form of the array arr[] is { 1, 2, 3, 4, 5, 6, 7, 8 }. Therefore, the 2nd smallest element in the array arr[] is 2.

Input: arr[] = { 10, 8, 6, 5, 2, 1, 13, 12 }, K = 3
Output: 5

 

Naive Approach: The simplest approach to solve the problem is to sort the given array arr[] in increasing order and print the Kth smallest element in the array. 



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

Alternative approach of the above solution: We can sort the array without using any sorting technique which will surely reduce the time complexity. We can just find the pivot point P in the array(around which the rotation occurs) using binary search and just reverse the two subarrays [0, P + 1] and [P + 1, N] using std::reverse() function in C++.  

Reversing the subarrays: After finding the pivot point P, just find the reverse of the two subarrays as following:  

std::reverse(arr, arr + P + 1);  

std::reverse(arr + P + 1, arr + N);  

And thus we get the sorted array and we can print the Kth smallest element as arr[K-1]

C++




// C++ program for the above approach.
#include <bits/stdc++.h>
using namespace std;
 
/* Function to get pivot. For array 4, 3, 2, 1, 6, 5
   it returns 3 (index of 1) */
int findPivot(int arr[], int low, int high)
{
    // base cases
    if (high < low)
        return -1;
    if (high == low)
        return low;
 
    int mid = (low + high) / 2; /*low + (high - low)/2;*/
    if (mid < high && arr[mid] < arr[mid + 1])
        return mid;
 
    if (mid > low && arr[mid] > arr[mid - 1])
        return (mid - 1);
 
    if (arr[low] <= arr[mid])
        return findPivot(arr, low, mid - 1);
 
    return findPivot(arr, mid + 1, high);
}
 
// Driver Code
int main()
{
 
    // Given Input
    int arr[] = { 10, 8, 6, 5, 2, 1, 13, 12 };
    int N = sizeof(arr) / sizeof(arr[0]);
    int K = 3;
 
    // Function Call
    int P = findPivot(arr, 0, N - 1);
 
    // reversing first subarray
    reverse(arr, arr + P + 1);
 
    // reversing second subarray
    reverse(arr + P + 1, arr + N);
    // printing output
    cout << arr[K - 1];
    return 0;
}
 
// This code is contributed by Pranjay Vats
Output
5

Time Complexity: O(N)

Auxiliary Space: O(1)

Efficient Approach: The optimal idea is based on the observation that the Rth element is the smallest element because the elements in the range [1, R] are reversed. Now, if the random index is R, it means subarray [1, R] and [R + 1, N] are sorted in decreasing order. Therefore, the task reduceS to finding the value of R which can be obtained using binary search. Finally, print the Kth smallest element.

Follow the steps below to solve the problem:

  • Initialize l as 1 and h as N to store the boundary elements index of the search space for the binary search.
  • Loop while the value of l+1 < h
    • Store the middle element in a variable, mid as (l+h)/2.
    • If arr[l] ≥ arr[mid]. If it is true then check on the right side of mid by updating l to mid.
    • Otherwise, update r to mid.
  • Now after finding R, if K ≤  R, then the answer is arr[R-K+1]. Otherwise, arr[N-(K-R)+1].

Below is the implementation of the above approach: 

C++




// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to find the Kth element in a
// sorted and rotated array at random point
int findkthElement(vector<int> arr, int n, int K)
{
     
    // Set the boundaries for binary search
    int l = 0;
    int h = n - 1, r;
 
    // Apply binary search to find R
    while (l + 1 < h)
    {
         
        // Initialize the middle element
        int mid = (l + h) / 2;
         
        // Check in the right side of mid
        if (arr[l] >= arr[mid])
            l = mid;
             
        // Else check in the left side
        else
            h = mid;
    }
     
    // Random point either l or h
    if (arr[l] < arr[h])
        r = l;
    else
        r = h;
     
    // Return the kth smallest element
    if (K <= r + 1)
        return arr[r + 1 - K];
    else
        return arr[n - (K - (r + 1))];
}
 
// Driver Code
int main()
{
     
    // Given Input
    vector<int> arr = { 10, 8, 6, 5, 2, 1, 13, 12 };
    int n = arr.size();
    int K = 3;
     
    // Function Call
    cout << findkthElement(arr, n, K);
}
 
// This code is contributed by mohit kumar 29

Java




// Java program for the above approach
class GFG{
     
// Function to find the Kth element in a
// sorted and rotated array at random point
public static int findkthElement(int arr[], int n, int K)
{
     
    // Set the boundaries for binary search
    int l = 0;
    int h = n - 1, r;
 
    // Apply binary search to find R
    while (l + 1 < h)
    {
         
        // Initialize the middle element
        int mid = (l + h) / 2;
         
        // Check in the right side of mid
        if (arr[l] >= arr[mid])
            l = mid;
             
        // Else check in the left side
        else
            h = mid;
    }
     
    // Random point either l or h
    if (arr[l] < arr[h])
        r = l;
    else
        r = h;
     
    // Return the kth smallest element
    if (K <= r + 1)
        return arr[r + 1 - K];
    else
        return arr[n - (K - (r + 1))];
}
 
// Driver Code
public static void main(String args[])
{
     
    // Given Input
    int []arr = { 10, 8, 6, 5, 2, 1, 13, 12 };
    int n = arr.length;
    int K = 3;
     
    // Function Call
    System.out.println(findkthElement(arr, n, K));
}
}
 
// This code is contributed by SoumikMondal

Python3




# Python program for the above approach
 
# Function to find the Kth element in a
# sorted and rotated array at random point
def findkthElement(arr, n, K):
   
      # Set the boundaries for binary search
    l = 0
    h = n-1
 
    # Apply binary search to find R
    while l+1 < h:
       
          # Initialize the middle element
        mid = (l+h)//2
 
        # Check in the right side of mid
        if arr[l] >= arr[mid]:
            l = mid
 
        # Else check in the left side
        else:
            h = mid
 
    # Random point either l or h
    if arr[l] < arr[h]:
        r = l
    else:
        r = h
 
    # Return the kth smallest element
    if K <= r+1:
        return arr[r+1-K]
    else:
        return arr[n-(K-(r+1))]
 
 
# Driver Code
if __name__ == "__main__":
   
      # Given Input
    arr = [10, 8, 6, 5, 2, 1, 13, 12]
    n = len(arr)
    K = 3
     
    # Function Call
    print(findkthElement(arr, n, K) )

C#




using System.IO;
using System;
 
class GFG {
 
    // Function to find the Kth element in a
    // sorted and rotated array at random point
    public static int findkthElement(int[] arr, int n,
                                     int K)
    {
 
        // Set the boundaries for binary search
        int l = 0;
        int h = n - 1, r;
 
        // Apply binary search to find R
        while (l + 1 < h) {
 
            // Initialize the middle element
            int mid = (l + h) / 2;
 
            // Check in the right side of mid
            if (arr[l] >= arr[mid])
                l = mid;
 
            // Else check in the left side
            else
                h = mid;
        }
 
        // Random point either l or h
        if (arr[l] < arr[h])
            r = l;
        else
            r = h;
 
        // Return the kth smallest element
        if (K <= r + 1)
            return arr[r + 1 - K];
        else
            return arr[n - (K - (r + 1))];
    }
 
    static void Main()
    {
        // Given Input
        int[] arr = { 10, 8, 6, 5, 2, 1, 13, 12 };
        int n = arr.Length;
        int K = 3;
 
        // Function Call
        Console.WriteLine(findkthElement(arr, n, K));
    }
}
 
// This code is contributed by abhinavjain194.

Javascript




<script>
 
      // Function to find the Kth element in a
      // sorted and rotated array at random point
      function findkthElement(arr, n, K) {
        // Set the boundaries for binary search
        var l = 0;
        var h = n - 1,
          r;
 
        // Apply binary search to find R
        while (l + 1 < h) {
          // Initialize the middle element
          var mid = parseInt((l + h) / 2);
 
          // Check in the right side of mid
          if (arr[l] >= arr[mid]) l = mid;
          // Else check in the left side
          else h = mid;
        }
 
        // Random point either l or h
        if (arr[l] < arr[h]) r = l;
        else r = h;
 
        // Return the kth smallest element
        if (K <= r + 1) return arr[r + 1 - K];
        else return arr[n - (K - (r + 1))];
      }
 
      // Given Input
      var arr = [10, 8, 6, 5, 2, 1, 13, 12];
      var n = arr.length;
      var K = 3;
 
      // Function Call
      document.write(findkthElement(arr, n, K));
       
</script>
Output
5

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

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