Find given occurrences of Mth most frequent element of Array

// C# program to implement above approach
using System;
using System.Collections;
using System.Collections.Generic;

class GFG
{

    // Function for finding indexes
    public static List<int> findindexes(List<int> arr, int M, List<int> query)
    {
        // Unordered map to store the unique
        // element as keys and vector as value
        // to store indexes
        Dictionary<int, List<int>> fre = new Dictionary<int, List<int>>();
        int n = arr.Count;

        // Ans vector to store and return
        // indexes occurred
        List<int> ans = new List<int>();

        int i;

        for (i = 0 ; i < n ; i++) {

            // For each key push that particular
            // index+1 1-bases indexing
            if(!fre.ContainsKey(arr[i])){
                fre.Add(arr[i], new List<int>());
            }
            fre[arr[i]].Add(i + 1);
        }

        SortedSet<pair> st = new SortedSet<pair>(new Comp());
        foreach(KeyValuePair<int, List<int>> it in fre){
            st.Add(new pair(it.Value.Count, it.Key));
        }

        int maxelement = 0;
        i = 0;
        foreach (pair it in st) {
            if(i >= M) break;
            maxelement = it.second;
            i++;
        }

        for (i = 0 ; i < query.Count ; i++) {

            // If kth index is not available
            if(!fre.ContainsKey(maxelement))
            {
                ans.Add(-1);
            }
            else
            {
                if (fre[maxelement].Count < query[i])
                    ans.Add(-1);
                else {

                    // Storing the indexes
                    // of maxfre element
                    ans.Add(fre[maxelement][query[i] - 1]);
                }
            }
        }

        return ans;
    }


    // Driver Code
    public static void Main(string[] args){
        
        // Input array
        List<int> arr = new List<int>{
            1, 2, 20, 8, 8, 1, 2, 5, 8, 0, 6, 8, 2
        };

        int M = 1;
        List<int> query = new List<int>{
            100, 4, 2
        };

        // Function call
        List<int> ans = findindexes(arr, M, query);
        for (int i = 0 ; i < ans.Count ; i++) {
            Console.Write(ans[i] + " ");
        }

    }
}

public class pair{
    public int first{ get; set; }
    public int  second{ get; set; }
    public pair(int first,int second){
        this.first = first;
        this.second = second;
    }
}

class Comp : IComparer<pair>{
    public int Compare(pair o2,pair o1){
        if(o1.first == o2.first){
            return o1.second - o2.second;
        }
        return o1.first - o2.first;
    }
}