Design a stack which can give maximum frequency element

Given N elements and the task is to implement a stack which removes and returns the maximum frequency element on every pop operation. If there’s a tie in the frequency then the topmost highest frequency element will be returned.

Examples:

Input:
push(4) 8
push(6) 6
push(7) 7
push(6) 6
push(8); 4

Output:
pop() -> returns 6, as 6 is the most frequent (frequency of 6 = 2 ).
pop() -> returns 8 (6 also has the highest frequency but it is not the topmost)

Approach: Maintaining two HashMap, one is frequency HashMap which maps elements to their frequencies and other is setMap which maps all the element with same frequency in one group (Stack).
FrequencyStack has 2 functions:

  1. push(int x): map the element (x) with frequency HashMap and update the maxfreq variable ( i.e. holds the maximum frequency till now ). setMap maintains a stack which contains all the elements with same frequency.
  2. pop(): First get the maxfreq element from setMap and then decrement the frequency of the popped element. After popping, if the stack becomes empty then decrement the maxfreq.

Below is the implementation of the above approach:

C++

// C++ implementation of the approach
#include
using namespace std;

// freqMap is to map element to its frequency
map freqMap;

// setMap is to map frequency to the
// element list with this frequency
map > setMap;

// Variable which stores maximum frequency
// of the stack element
int maxfreq = 0;

// Function to insert x in the stack
void push(int x)
{

// Frequency of x
int freq = freqMap[x] + 1;

// Mapping of x with its frequency
freqMap[x]= freq;

// Update maxfreq variable
if (freq > maxfreq)
maxfreq = freq;

// Map element to its frequency list
// If given frequency list doesn’t exist
// make a new list of the required frequency
setMap[freq].push(x);
}

// Function to remove maximum frequency element
int pop()
{

// Remove element from setMap
// from maximum frequency list
int top = setMap[maxfreq].top();
setMap[maxfreq].pop();

// Decrement the frequency of the popped element
freqMap[top]–;

// If whole list is poppped
// then decrement the maxfreq
if (setMap[maxfreq].size() == 0)
maxfreq–;
return top;
}

// Driver code
int main()
{

// Push elements to the stack
push(4);
push(6);
push(7);
push(6);
push(8);

// Pop elements
cout << (pop()) << "\n" ; cout << (pop()); return 0; } // This code is contributed by Arnab Kundu [tabby title="Java"]

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// Java implementation of the approach
import java.util.*;
  
public class freqStack {
  
    // freqMap is to map element to its frequency
    static Map<Integer, Integer> freqMap = new HashMap<>();
  
    // setMap is to map frequency to the
    // element list with this frequency
    static Map<Integer, Stack<Integer> > setMap = new HashMap<>();
  
    // Variable which stores maximum frequency
    // of the stack element
    static int maxfreq = 0;
  
    // Function to insert x in the stack
    public static void push(int x)
    {
  
        // Frequency of x
        int freq = freqMap.getOrDefault(x, 0) + 1;
  
        // Mapping of x with its frequency
        freqMap.put(x, freq);
  
        // Update maxfreq variable
        if (freq > maxfreq)
            maxfreq = freq;
  
        // Map element to its frequency list
        // If given frequency list doesn't exist
        // make a new list of the required frequency
        setMap.computeIfAbsent(freq, z -> new Stack()).push(x);
    }
  
    // Function to remove maximum frequency element
    public static int pop()
    {
  
        // Remove element from setMap
        // from maximum frequency list
        int top = setMap.get(maxfreq).pop();
  
        // Decrement the frequency of the popped element
        freqMap.put(top, freqMap.get(top) - 1);
  
        // If whole list is poppped
        // then decrement the maxfreq
        if (setMap.get(maxfreq).size() == 0)
            maxfreq--;
        return top;
    }
  
    // Driver code
    public static void main(String[] args)
    {
  
        // Push elements to the stack
        push(4);
        push(6);
        push(7);
        push(6);
        push(8);
  
        // Pop elements
        System.out.println(pop());
        System.out.println(pop());
    }
}

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Output:

6
8


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