Sort elements by frequency using STL

Given an array of integers, sort the array according to frequency of elements. If frequencies of two elements are same, print them in increasing order. Examples:

Input : arr[] = {2, 3, 2, 4, 5, 12, 2, 3, 3, 3, 12}
Output : 3 3 3 3 2 2 2 12 12 4 5
Explanation :
No. Freq
2  : 3
3  : 4
4  : 1
5  : 1
12 : 2

We have discussed different approaches in below posts : Sort elements by frequency | Set 1 Sort elements by frequency | Set 2 We can solve this problem using map and pairs. Initially we create a map such that map[element] = freq. Once we are done building the map, we create an array of pairs. A pair which stores elements and their corresponding frequency will be used for the purpose of sorting. We write a custom compare function which compares two pairs firstly on the basis of freq and if there is a tie on the basis of values. 

Below is its c++ implementation : 

C++

// C++ program to sort elements by frequency using
// STL
#include <bits/stdc++.h>
using namespace std;

// function to compare two pairs for inbuilt sort
bool compare(pair<int,int> &p1,
            pair<int, int> &p2)
{
    // If frequencies are same, compare
    // values
    if (p1.second == p2.second)
        return p1.first < p2.first;
    return p1.second > p2.second;
}

// function to print elements sorted by freq
void printSorted(int arr[], int n)
{
    // Store items and their frequencies
    map<int, int> m;
    for (int i = 0; i < n; i++)
        m[arr[i]]++;

    // no of distinct values in the array
    // is equal to size of map.
    int s = m.size();

    // an array of pairs
    pair<int, int> p[s];

    // Fill (val, freq) pairs in an array
    // of pairs.
    int i = 0;
    for (auto it = m.begin(); it != m.end(); ++it)
        p[i++] = make_pair(it->first, it->second);

    // sort the array of pairs using above
    // compare function.
    sort(p, p + s, compare);

    cout << "Elements sorted by frequency are: ";
    for (int i = 0; i < s; i++)
    {
        int freq = p[i].second;
        while (freq--)
            cout << p[i].first << " ";
    }
}

// driver program
int main()
{
    int arr[] = {2, 3, 2, 4, 5, 12, 2, 3,
                3, 3, 12};
    int n = sizeof(arr)/ sizeof(arr[0]);
    printSorted(arr, n);
    return 0;
}

Java

// Java program to sort elements by frequency using
import java.util.*;

public class Main
{

    // function to compare two pairs for inbuilt sort
    static boolean compare(Map.Entry<Integer, Integer> p1,
                           Map.Entry<Integer, Integer> p2)
    {
        // If frequencies are same, compare values
        if (p1.getValue().equals(p2.getValue()))
            return p1.getKey() < p2.getKey();
        return p1.getValue() > p2.getValue();
    }

    // function to print elements sorted by freq
    static void printSorted(int[] arr, int n)
    {
        // Store items and their frequencies
        Map<Integer, Integer> m
            = new HashMap<Integer, Integer>();
        for (int i = 0; i < n; i++)
            m.put(arr[i], m.getOrDefault(arr[i], 0) + 1);

        // no of distinct values in the array
        // is equal to size of map.
        int s = m.size();

        // an array of Map.Entry pairs
        List<Map.Entry<Integer, Integer> > list
            = new ArrayList<Map.Entry<Integer, Integer> >(
                m.entrySet());

        // sort the list of Map.Entry pairs using above
        // compare function.
        Collections.sort(
            list, (p1, p2) -> compare(p1, p2) ? -1 : 1);

        System.out.print(
            "Elements sorted by frequency are: ");
        for (Map.Entry<Integer, Integer> entry : list) {
            int freq = entry.getValue();
            while (freq-- > 0)
                System.out.print(entry.getKey() + " ");
        }
    }

    // driver program
    public static void main(String[] args)
    {
        int[] arr = { 2, 3, 2, 4, 5, 12, 2, 3, 3, 3, 12 };
        int n = arr.length;
        printSorted(arr, n);
    }
}

Python3

from collections import Counter

# function to compare two elements for inbuilt sort
def compare(x, y):
    # If frequencies are same, compare
    # values
    if x[1] == y[1]:
        return x[0] < y[0]
    return x[1] > y[1]

# function to print elements sorted by freq
def printSorted(arr, n):
    # Store items and their frequencies
    m = Counter(arr)

    # no of distinct values in the array
    # is equal to size of dictionary.
    s = len(m)

    # an array of pairs
    p = [(k, v) for k, v in m.items()]

    # sort the array of pairs using above
    # compare function.
    p = sorted(p, key=lambda x: compare(x, x))

    print("Elements sorted by frequency are: ", end='')
    for i in range(s):
        freq = p[i][1]
        while freq > 0:
            print(p[i][0], end=' ')
            freq -= 1

# driver program
arr = [2, 3, 2, 4, 5, 12, 2, 3, 3, 3, 12]
n = len(arr)
printSorted(arr, n)

# This code is contributed by shiv1o43g

C#

using System;
using System.Collections.Generic;
using System.Linq;

public class MainClass
{
  // function to compare two pairs for inbuilt sort
  static bool compare(KeyValuePair<int, int> p1, KeyValuePair<int, int> p2)
  {
    // If frequencies are same, compare values
    if (p1.Value == p2.Value)
      return p1.Key < p2.Key;
    return p1.Value > p2.Value;
  }

  // function to print elements sorted by freq
  static void printSorted(int[] arr, int n)
  {
    // Store items and their frequencies
    Dictionary<int, int> m = new Dictionary<int, int>();
    for (int i = 0; i < n; i++)
    {
      if (m.ContainsKey(arr[i]))
        m[arr[i]]++;
      else
        m[arr[i]] = 1;
    }

    // no of distinct values in the array
    // is equal to size of dictionary.
    int s = m.Count;

    // an array of KeyValuePair pairs
    List<KeyValuePair<int, int>> list =
      new List<KeyValuePair<int, int>>(m);

    // sort the list of KeyValuePair pairs using above
    // compare function.
    list.Sort((p1, p2) => compare(p1, p2) ? -1 : 1);

    Console.Write("Elements sorted by frequency are: ");
    foreach (KeyValuePair<int, int> entry in list)
    {
      int freq = entry.Value;
      while (freq-- > 0)
        Console.Write(entry.Key + " ");
    }
  }

  // driver program
  public static void Main(string[] args)
  {
    int[] arr = { 2, 3, 2, 4, 5, 12, 2, 3, 3, 3, 12 };
    int n = arr.Length;
    printSorted(arr, n);
  }
}

Javascript

// function to compare two pairs for sorting
function compare(p1, p2)
{

  // If frequencies are same, compare values
  if (p1[1] == p2[1]) {
    return p1[0] < p2[0];
  }
  return p1[1] > p2[1];
}

// function to print elements sorted by frequency
function printSorted(arr)
{

  // Store items and their frequencies
  let m = new Map();
  for (let i = 0; i < arr.length; i++) {
    if (m.has(arr[i])) {
      m.set(arr[i], m.get(arr[i]) + 1);
    } else {
      m.set(arr[i], 1);
    }
  }

  // no of distinct values in the array
  // is equal to size of map.
  let s = m.size;

  // an array of pairs
  let p = new Array(s);

  // Fill (val, freq) pairs in an array of pairs.
  let i = 0;
  for (let [key, value] of m) {
    p[i] = [key, value];
    i++;
  }

  // sort the array of pairs using insertion sort algorithm
  for (let i = 1; i < s; i++) {
    let j = i - 1;
    let key = p[i];
    while (j >= 0 && compare(p[j], key)) {
      p[j + 1] = p[j];
      j = j - 1;
    }
    p[j + 1] = key;
  }

  process.stdout.write("Elements sorted by frequency are: ");
  for (let i = s-1; i >= 0; i--) {
    let freq = p[i][1];
    while (freq--) {
      process.stdout.write(p[i][0] + " ");
    }
  }
}

// driver program
let arr = [2, 3, 2, 4, 5, 12, 2, 3, 3, 3, 12];
printSorted(arr);

Output

Elements sorted by frequency are: 3 3 3 3 2 2 2 12 12 4 5 

Time Complexity : O(n Log n) 

Space Complexity: O(n)
The above algorithm requires O(n) space for the hash map and the array of pairs.

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