Data structure used:

1. Set (ordered in decreasing fashion)
2. Map (ordered in decreasing fashion)

Algorithm:

1. Need to store the marks in sorted way descending order.
2. The marks are key and we need to map student indices to the key value. While mapping indices needed to be mapped in ascending fashion.
3. Print indices for K keys ( marks value already sorted in decreasing fashion, thus top K keys are top K marks).

Implementation with the data structures used:

1. Declare records as a map.

   map<int, vector<int>, greater <int>> records;
   

   map<> = ordered map usually ordered in ascending fashion as per key value greater <int> is used to order in descending fashion.
   Here our key is integer type which maps to a vector of integer. Clearly the key is marks & which maps to a list of indices of students.
2. Declare numbers as a set.

   set<int, greater<int>> numbers; 
   

   set<> = ordered set usually ordered in ascending fashion as per element value greater <int> is used to order in descending fashion.
   Here the elements are the marks which are stored in sorted descending fashion.
3. After completion of the input taking, both records&numbers are filled with datas as per mentioned previously.

Let's consider the above input example:

    K=2
    N=10
    Marks are:
    97 84 82 89 84 97 95 95 84 86    

So after completion of input taking:

Records looks like:

Numbers looks like:

K=2, thus we need to print indices only for marks 97, 95

Thus the indices to be print are: 0 5 6 7

So to print:

For i =0: K
	Set iterator tonumbers.begin() //points to 97
	Advance iterator by i; //to point at ith mark from the top
	Print the vector list associated with the key (marks) pointed to.
END FOR

C++ implementation for Toppers of Class

#include <bits/stdc++.h>
using namespace std;

int main()
{
	int n,k;
	//n is no of students, k is the input K

	cout<<"enter no of student\n";
	scanf("%d",&n);
	
	map<int,vector<int>,greater <int>> records;//declare records
	set<int,greater<int>> numbers;//declare numbers
	
	int no;
	cout<<"enter the marks of the students\n";
	for(int i=0;i<n;i++){
		cin>>no;
		//for key value build the vector list
		records[no].push_back(i); 
		//to avoid duplicate
		if(numbers.find(no)==numbers.end()) 
			numbers.insert(no); //insert marks to set
	}
	
	cout<<"enter K\n";
	cin>>k; //input K

	cout<<"Toppers are: ";
	//printing the indices
	for(int i=0;i<k;i++){ 
		auto ij=numbers.begin();
		advance(ij,i);
		//printing the associated vector
		for(auto it=records[*ij].begin();it!=records[*ij].end();it++){ 
			printf("%d ",*it);
		}
	}

	cout<<endl;
	
	return 0;
}

Output

enter no of student
10
enter the marks of the students
97 84 82 89 84 97 95 95 84 86
enter K
2
Toppers are: 0 5 6 7