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K’th Smallest/Largest Element using STL

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  • Difficulty Level : Easy
  • Last Updated : 21 Aug, 2022
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Given an array and a number k where k is smaller than size of array, we need to find the k’th smallest element in the given array. Examples:

Input : arr[] = {7, 10, 4, 3, 20, 15}
            k = 2
Output : 4
Smallest element is 3. Second smallest
is 4.

Input : arr[] = {7, 10, 4, 3, 3, 15}
            k = 2
Output : 4
Even if there are more than one occurrences
of 3, answer should be 4.

Input :arr[] = {7, 10, 4, 3, 20, 15}
          k = 4
Output : 10

We use set in C++ STL. 1) Insert all elements into a set. 2) Traverse the set and print k-th element. 

Implementation:

CPP




// STL based C++ program to find k-th smallest
// element.
#include <bits/stdc++.h>
using namespace std;
 
int kthSmallest(int arr[], int n, int k)
{
 // Insert all elements into the set
 set<int> s;
 for (int i = 0; i < n; i++)
  s.insert(arr[i]);
 
 // Traverse set and print k-th element
 auto it = s.begin();
 for (int i = 0; i < k - 1; i++)
  it++;
 return *it;
}
 
int main()
{
 int arr[] = { 12, 3, 5, 7, 3, 19 };
 int n = sizeof(arr) / sizeof(arr[0]), k = 2;
 cout << "K'th smallest element is "
  << kthSmallest(arr, n, k);
 return 0;
}


Output

K'th smallest element is 5

Time complexity: O(n Log n). Note that set in STL uses a self-balancing BST internally and therefore time complexity of search and insert operations is O(log n). 
Auxiliary Space: O(n) where n is size of array, since n extra space has been taken.

Related Posts : 
K’th Smallest/Largest Element in Unsorted Array | Set 1
K’th Smallest/Largest Element in Unsorted Array | Set 2 (Expected Linear Time K’th Smallest/Largest Element in Unsorted Array | Set 3 (Worst Case Linear Time)

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