Find the Number of Maximum Product Quadruples

Given an array of N positive elements find the number of quadruple, (i, j, k, m) such that i < j < k < m such that the product aiajakam is the maximum possible

Examples:

Input : N = 7, arr = {1, 2, 3, 3, 3, 3, 5}  
Output : 4 
 Explanation 
The maximum quadruple product possible is 135, which can be 
achieved by the following quadruples {i, j, k, m} such that aiajakam = 135:
1) a3a4a5a7
2) a3a4a6a7
3) a4a5a6a7
4) a3a5a6a7

Input : N = 4, arr = {1, 5, 2, 1} 
Output : 1 
 Explanation 
The maximum quadruple product possible is 10, which can be 
achieved by the following quadruple {1, 2, 3, 4} as a1a2a3a4 = 10


Brute Force: O(n4)
Generate all possible quadruples and count the quadruples giving the maximum product

Optimized Solution:
It is easy to see that the product of the four largest numbers would be maximum. So, the problem can now be reduced to finding the number of ways of selecting the four largest elements. To do so, maintain a frequency array which stores the frequency of each element of the array.
Suppose the largest element is X with frequency FX, then if the frequency of this element is >= 4, it is best suited to select the four elements as X, X, X as this given a maximum product and the number of ways to do so are FX C 4
and if the frequency is less than 4, the number of ways to select this is 1 and now the required number of elements are 4 – FX. For the second element, say Y, the number of ways are: FX C remaining_choices. Remaining choices denotes the number of additional elements we need to select after selecting the first element. If at any time remaining_choices = 0, it means the quadruples are selected, so we can stop the algorithm

C++

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// CPP program to find the number of Quadruples
// having maximum product
#include <bits/stdc++.h>
using namespace std;
  
// Returns the number of ways to select r objects
// out of available n choices
int NCR(int n, int r)
{
    int numerator = 1;
    int denominator = 1;
  
    // ncr = (n * (n - 1) * (n - 2) * .....
    // ... (n - r + 1)) / (r * (r - 1) * ... * 1)
    while (r > 0) {
        numerator *= n;
        denominator *= r;
        n--;
        r--;
    }
  
    return (numerator / denominator);
}
  
// Returns the number of quadruples having maximum product
int findWays(int arr[], int n)
{
    // stores the frequency of each element
    map<int, int> count;
  
    if (n < 4)
        return 0;
  
    for (int i = 0; i < n; i++) {
        count[arr[i]]++;
    }
  
    // remaining_choices denotes the remaining 
    // elements to select inorder to form quadruple
    int remaining_choices = 4;
    int ans = 1;
  
    // traverse the elements of the map in reverse order
    for (auto iter = count.rbegin(); iter != count.rend(); ++iter) {
        int number = iter->first;
        int frequency = iter->second;
  
        // If Frequeny of element < remaining choices,
        // select all of these elements, else select only 
        // the number of elements required
        int toSelect = min(remaining_choices, frequency);
        ans = ans * NCR(frequency, toSelect);
  
        // Decrement remaining_choices acc to the number
        // of the current elements selected
        remaining_choices -= toSelect;
  
        // if the quadruple is formed stop the algorithm
        if (!remaining_choices) {
            break;
        }
    }
    return ans;
}
  
// Driver Code
int main()
{
    int arr[] = { 1, 2, 3, 3, 3, 5 };
    int n = sizeof(arr) / sizeof(arr[0]);
  
    int maxQuadrupleWays = findWays(arr, n);
    cout << maxQuadrupleWays;
  
    return 0;
}

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// Java program to find the number of Quadruples
// having maximum product
import java.util.*;
class Solution
{
// Returns the number of ways to select r objects
// out of available n choices
static int NCR(int n, int r)
{
    int numerator = 1;
    int denominator = 1;
  
    // ncr = (n * (n - 1) * (n - 2) * .....
    // ... (n - r + 1)) / (r * (r - 1) * ... * 1)
    while (r > 0) {
        numerator *= n;
        denominator *= r;
        n--;
        r--;
    }
  
    return (numerator / denominator);
}
  
// Returns the number of quadruples having maximum product
static int findWays(int arr[], int n)
{
    // stores the frequency of each element
    HashMap<Integer,Integer> count= new HashMap<Integer,Integer>();
  
    if (n < 4)
        return 0;
  
    for (int i = 0; i < n; i++) {
        count.put(arr[i],(count.get(arr[i])==null?0🙁int)count.get(arr[i])));
    }
  
    // remaining_choices denotes the remaining 
    // elements to select inorder to form quadruple
    int remaining_choices = 4;
    int ans = 1;
      
    // Getting an iterator 
        Iterator hmIterator = count.entrySet().iterator(); 
    
        while (hmIterator.hasNext()) { 
            Map.Entry mapElement = (Map.Entry)hmIterator.next();
            int number =(int) mapElement.getKey();
            int frequency =(int)mapElement.getValue();
              
              // If Frequeny of element < remaining choices,
        // select all of these elements, else select only 
        // the number of elements required
        int toSelect = Math.min(remaining_choices, frequency);
        ans = ans * NCR(frequency, toSelect);
  
        // Decrement remaining_choices acc to the number
        // of the current elements selected
        remaining_choices -= toSelect;
  
        // if the quadruple is formed stop the algorithm
        if (remaining_choices==0) {
            break;
        }
        }
     
    return ans;
}
  
// Driver Code
public static void main(String args[])
{
    int arr[] = { 1, 2, 3, 3, 3, 5 };
    int n = arr.length;
  
    int maxQuadrupleWays = findWays(arr, n);
    System.out.print( maxQuadrupleWays);
}
}
//contributed by Arnab Kundu

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Output:

1

Time Complexity: O(NlogN), where N is sizeof the array



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