Generate a Number in Decreasing order of Frequencies of characters of a given String

// java Program to implement
// the above approach
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class FormString {
    static class Pair implements Comparable<Pair> {
        char first;
        int second;

        Pair(char first, int second) {
            this.first = first;
            this.second = second;
        }
        // custom comparator for sorting
        @Override
        public int compareTo(Pair o) {
             // is frequency same
            if (this.second == o.second) {
                // Character with lower ASCII
                // value appears first
                return this.first - o.first;
            }
            // Otherwise character with higher
            // frequency appears first
            return o.second - this.second;
        }
    }
// Function to sort map accordingly
    static String sort(Map<Character, Integer> map) {
        // Declaring vector of pairs
        List<Pair> list = new ArrayList<>();
        // Traversing map and pushing
        // pairs to List Integerface
        for (Map.Entry<Character, Integer> entry : map.entrySet()) {
            list.add(new Pair(entry.getKey(), entry.getValue()));
        }
        // Sorting using Collections sort
        Collections.sort(list);
        StringBuilder sb = new StringBuilder();
        for (Pair pair : list) {
             // Get the possible digit
            // from assigned value and
            // Ensuring k does not exceed 9 
            int k = (pair.first - 'a' + 1) % 10;
            // append each digit
            sb.append(k);
        }
        // Returning final result
        return sb.toString();
    }

    static String formString(String s) {
        // map are used to store freq
        Map<Character, Integer> map = new HashMap<>();
        for (int i = 0; i < s.length(); i++) {
            map.put(s.charAt(i), map.getOrDefault(s.charAt(i), 0) + 1);
        }
        // sort map and return 
        return sort(map);
    }
   // Main function (Driver code)
    public static void main(String[] args) {
        String Str = "akkzzz";
        System.out.println(formString(Str));
    }
}

import java.util.*;

public class Main {
    // Pair class implementation
    static class Pair<K, V> {
        public K key;
        public V value;

        public Pair(K key, V value) {
            this.key = key;
            this.value = value;
        }

        public K getKey() {
            return key;
        }

        public V getValue() {
            return value;
        }
    }

    public static String formString(String s) {
        // Step 1: Initialize count array
        int[] count = new int[26];
        for (int i = 0; i < s.length(); i++) {
            count[s.charAt(i) - 'a']++;
        }

        // Step 2: Create list of pairs
        List<Pair<Character, Integer>> v = new ArrayList<>();
        for (int i = 0; i < 26; i++) {
            if (count[i] > 0) {
                v.add(new Pair<>((char)('a' + i), count[i]));
            }
        }

        // Step 3: Sort list of pairs
        Collections.sort(v, new Comparator<Pair<Character, Integer>>() {
            @Override
            public int compare(Pair<Character, Integer> p1, Pair<Character, Integer> p2) {
                if (p1.getValue().equals(p2.getValue())) {
                    return Character.compare(p1.getKey(), p2.getKey());
                }
                return Integer.compare(p2.getValue(), p1.getValue());
            }
        });

        // Step 4: Generate result string
        StringBuilder res = new StringBuilder();
        for (Pair<Character, Integer> p : v) {
            int digit = (p.getKey() - 'a' + 1) % 10;
            res.append(digit);
        }

        // Step 5: Return result string
        return res.toString();
    }

    public static void main(String[] args) {
        int N = 6;
        String Str = "akkzzz";
        System.out.println(formString(Str)); // Output: 611
    }
}