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k size subsets with maximum difference d between max and min

  • Difficulty Level : Medium
  • Last Updated : 25 Aug, 2021

C++


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// C++ code to find no. of subsets with
// maximum difference d between max and
#include <bits/stdc++.h>
using namespace std;
 
// function to calculate factorial of a numb
int fact(int i)
{
 
    if (i == 0)
        return 1;
    return i * fact(i - 1);
}
 
int ans(int a[], int n, int k, int x)
{
    if (k > n || n < 1)
        return 0;
 
    sort(a, a + n);
    int count = 0;
    int j = 1;
    int i = 0;
    int kfactorial = fact(k);
    while (j <= n) {
        while (j < n && a[j] - a[i] <= x) {
            j++;
        }
        if ((j - i) >= k) {
            count = count
                    + fact(j - i)
                          / (kfactorial * fact(j - i - k));
        }
        else {
            i++;
            j++;
            continue;
        }
        if (j == n)
            break;
        while (i < j && a[j] - a[i] > x) {
            i++;
        }
        if ((j - i) >= k) {
            count = count
                    - fact(j - i)
                          / (kfactorial * fact(j - i - k));
        }
    }
 
    return count;
}
 
// driver program to test the above
// function
int main()
{
    int arr[] = { 1, 12, 9, 2, 4, 2, 5, 8, 4, 6 },
    k = 3,x = 5;
    int n = sizeof(arr) / sizeof(arr[0]);
    cout << ans(arr, n, k, x);
    return 0;
}
// This code is contributed by Vishakha Chauhan
Output
52

Given an array and two integers k and d, find the number of subsets of this array of size k, where difference between the maximum and minimum number of the subset is atmost d.
Examples: 
 

Input : a[] = [5, 4, 2, 1, 3],
        k = 3, d = 5 
Output : 10
Explanation:
{1,2,3}, {1,2,4}, {1,2,5}, {1,3,4}, {1,3,5}, 
{1,4,5}, {2,3,4}, {2,3,5}, {2,4,5}, {3,4,5}.
We can see each subset has atmost 
difference d=5 between the minimum
and maximum element of each subset.
No of such subsets = 10 

Input : a[] = [1, 2, 3, 4, 5, 6],
        k = 3, d = 5 
Output : 20

 



Naive approach: Finding all the subsets of size k and for each subset find the difference between maximum and minimum element. If the difference is less than or equal to d, count them. 
Efficient approach
1) Sorting: First sort the array in increasing order. Now, assume we want to find out for each ith element, the number of required subsets in which integer a[i] is present as the minimum element of that subset. The maximum in such a subset will never exceed a[i] + d . 
2) Find maximum index j : We can apply binary search over this array for each i, to find the maximum index j, such that a[j] <= a[i]+d . Now any subset that includes a[i] and any other elements from the range i+1…j will be a required subset as because element a[i] is the minimum of that subset, and the difference between any other element and a[i] is always less than equal to d. 
3) Apply basic combinatorics formula : Now we want to find the number of required subsets of size k. This will be by using the basic formula of combination when you have to select r items from given n numbers. In the same way we need to choose (k-1) numbers from (j-i) elements already including a[i] which is the minimum number in each subset. The sum of this procedure for each ith element will be the final answer. 
Here I have used a simple recursive way to find factorial of a number one can use dynamic programming as well to find it.
 

Illustration : 
Input : a = [5, 4, 2, 1, 3], 
k = 3, d = 5 
Output : 10 
Explanation: 
Sorted array in ascending order : [1, 2, 3, 4, 5]
For a[0] = 1 as minimum element 
No. of subset will be 6 which are {1, 2, 3}, {1, 2, 
4}, {1, 2, 5}, {1, 3, 4}, {1, 3, 5}, {1, 4, 5}.
For a[1] = 2 as minimum element 
No. of subset will be 3 which are {2, 3, 4}, {2, 
3, 5}, {2, 4, 5}
For a[2] = 3 as minimum element 
No. of subset will be 1 which is {3, 4, 5} 
No other subset of size k = 3 will be formed 
by taking a[3] = 4 or a[4] = 5 as minimum element

 

C++




// C++ code to find no. of subsets with
// maximum difference d between max and
// min of all K-size subsets function to
// calculate factorial of a number
#include <bits/stdc++.h>
using namespace std;
 
int fact (int n){
    if (n==0)
        return 1;
    else
        return n * fact(n-1);
}
 
// function to count ways to select r
// numbers from n given numbers
int findcombination (int n,int r){
    return( fact(n) / (fact(n - r) *
                        fact(r)));
}
 
// function to return the total number
// of required subsets :
// n is the number of elements in array
// d is the maximum difference between
// minimum and maximum element in each
// subset of size k
int find(int arr[], int n, int d, int k)
{
    sort(arr,arr+n);
    int ans = 0, end = n, co = 0,
        start = 0;
 
    // loop to traverse from 0-n
    for (int i = 0; i < n; i++) {
 
    int val = arr[i] + d;
     
    // binary search to get the position
    // which will be stored in start
     
    start = i;
    while (start < end - 1){
        int mid = (start + end) / 2;
 
        // if mid value greater than
        // arr[i]+d do search in
        // arr[start:mid]
        if (arr[mid] > val)
            end = mid;
 
        else
            start = mid + 1;
    }
     
    if (start != n and arr[start]
                       <= val)
            start += 1;
 
    int c = start-i;
 
    // if the numbers of elements 'c'
    // is greater or equal to the given
    // size k, then only subsets of
    // required size k can be formed
    if (c >= k){
        co += findcombination(c - 1, k - 1);}
    }
    return co;
}
 
// driver program to test the above
// function
int main()
{
    int arr[] = {1, 2, 3, 4, 5, 6},
        k = 3, d = 5;
    int n = sizeof(arr) / sizeof(arr[0]);
    cout << find(arr, n,d,k);
    return 0;
}
// This code is contributed by Prerna Saini

Java




// Java code to find no. of subsets
// with maximum difference d between
// max and min of all K-size subsets
import java.util.*;
 
class GFG {
 
// function to calculate factorial
// of a number
static int fact (int n){
    if (n==0)
        return 1;
    else
        return n * fact(n-1);
}
 
// function to count ways to select r
// numbers from n given numbers
static int findcombination(int n, int r){
    return( fact(n) / (fact(n - r) *
                           fact(r)));
}
 
// function to return the total number
// of required subsets :
// n is the number of elements in array
// d is the maximum difference between
// minimum and maximum element in each
// subset of size k
static int find(int arr[], int n, int d,
                            int k)
{
    Arrays.sort(arr);
    int ans = 0, end = n, co = 0,
        start = 0;
 
    // loop to traverse from 0-n
    for (int i = 0; i < n; i++) {
 
    int val = arr[i] + d;
     
    // binary search to get the position
    // which will be stored in start
    start=i;
    while (start < end - 1){
        int mid = (start + end) / 2;
 
        // if mid value greater than
        // arr[i]+d do search in
        // arr[start:mid]
        if (arr[mid] > val)
            end = mid;
        else
            start = mid+1;
        }
         
    if (start !=n && arr[start] <= val)
            start += 1;
 
        int c = start-i;
 
    // if the numbers of elements 'c' is
    // greater or equal to the given size k,
    // then only subsets of required size k
    // can be formed
    if (c >= k){
        co += findcombination(c - 1, k - 1);}
    }
     
    return co;
}
 
// driver program to test the above function
public static void main(String[] args)
{
    int arr[] = {1, 2, 3, 4, 5, 6}, k = 3,
        d = 5;
    int n = arr.length;
    System.out.println(find(arr, n,d,k));
}
}
// This code is contributed by Prerna Saini

Python




# Python code to find no. of subsets with maximum
# difference d between max and min of all K-size
# subsets function to calculate factorial of a
# number
def fact (n):
    if (n==0):
        return (1)
    else:
        return n * fact(n-1)
 
# function to count ways to select r numbers
# from n given numbers
def findcombination (n,r):
    return( fact(n)//(fact(n-r)*fact(r)))
 
# function to return the total number of required
# subsets :
# n is the number of elements in list l[0..n-1]
# d is the maximum difference between minimum and
#    maximum element in each subset of size k   
def find (a, n, d, k):
 
    # sort the list first in ascending order
    a.sort()
    (start, end, co) = (0, n, 0)
 
    for i in range(0, n):
        val = a[i]+ d
 
        # binary search to get the position
        # which will be stored in start
        # such that a[start] <= a[i]+d
        start = i
        while (start< end-1):
            mid = (start+end)//2
 
            # if mid value greater than a[i]+d
            # do search in l[start:mid]
            if (a[mid] > val):
                end = mid
 
            # if mid value less or equal to a[i]+d
            # do search in a[mid+1:end]
            else:
                start = mid+1
 
        if (start!=n and a[start]<=val):
            start += 1
 
        # count the numbers of elements that fall
        # in range i to start
        c = start-i
 
        # if the numbers of elements 'c' is greater
        # or equal to the given size k, then only
        # subsets of required size k can be formed
        if (c >= k):
            co += findcombination(c-1,k-1)
 
    return co
 
# Driver code
n = 6  # Number of elements
d = 5  # maximum diff
k = 3  # Size of subsets
print(find([1, 2, 3, 4, 5, 6], n, d, k)) 

C#




// C# code to find no. of subsets
// with maximum difference d between
// max and min of all K-size subsets
using System;
 
class GFG {
 
    // function to calculate factorial
    // of a number
    static int fact (int n)
    {
        if (n == 0)
            return 1;
        else
            return n * fact(n - 1);
    }
     
    // function to count ways to select r
    // numbers from n given numbers
    static int findcombination(int n, int r)
    {
        return( fact(n) / (fact(n - r) *
                               fact(r)));
    }
     
    // function to return the total number
    // of required subsets :
    // n is the number of elements in array
    // d is the maximum difference between
    // minimum and maximum element in each
    // subset of size k
    static int find(int []arr, int n, int d,
                                      int k)
    {
        Array.Sort(arr);
         
        //int ans = 0,
        int end = n, co = 0,
            start = 0;
     
        // loop to traverse from 0-n
        for (int i = 0; i < n; i++)
        {
            int val = arr[i] + d;
             
            // binary search to get the
            // position which will be
            // stored in start
            start = i;
            while (start < end - 1){
                int mid = (start + end) / 2;
         
                // if mid value greater than
                // arr[i]+d do search in
                // arr[start:mid]
                if (arr[mid] > val)
                    end = mid;
                else
                    start = mid+1;
                }
                 
            if (start !=n && arr[start] <= val)
                    start += 1;
         
                int c = start-i;
         
            // if the numbers of elements 'c' is
            // greater or equal to the given size k,
            // then only subsets of required size k
            // can be formed
            if (c >= k)
                co += findcombination(c - 1, k - 1);
        }
         
        return co;
    }
     
    // driver program to test the above function
    public static void Main()
    {
        int []arr = {1, 2, 3, 4, 5, 6};
        int k = 3;
        int d = 5;
        int n = arr.Length;
        Console.WriteLine(find(arr, n, d, k));
    }
}
 
// This code is contributed by anuj_67.

PHP




<?php
 
// Php code to find no. of subsets with
// maximum difference d between max and
// min of all K-size subsets function to
// calculate factorial of a number
  
function fact ($n){
    if ($n==0)
        return 1;
    else
        return $n * fact($n-1);
}
  
// function to count ways to select r
// numbers from n given numbers
function findcombination ($n,$r){
    return( fact($n) / (fact($n - $r) *
                        fact($r)));
}
  
// function to return the total number
// of required subsets :
// n is the number of elements in array
// d is the maximum difference between
// minimum and maximum element in each
// subset of size k
function find(&$arr, $n, $d, $k)
{
    sort($arr);
    $ans = 0;
    $end = $n;
    $co = 0;
    $start = 0;
  
    // loop to traverse from 0-n
    for ($i = 0; $i < $n; $i++) {
  
    $val = $arr[$i] + $d;
      
    // binary search to get the position
    // which will be stored in start
      
    $start = $i;
    while ($start < $end - 1){
        $mid = intval (($start + $end) / 2);
  
        // if mid value greater than
        // arr[i]+d do search in
        // arr[start:mid]
        if ($arr[$mid] > $val)
            $end = $mid;
  
        else
            $start = $mid + 1;
    }
      
    if ($start != $n && $arr[$start]
                       <= $val)
            $start += 1;
  
    $c = $start-$i;
  
    // if the numbers of elements 'c'
    // is greater or equal to the given
    // size k, then only subsets of
    // required size k can be formed
    if ($c >= $k){
        $co += findcombination($c - 1, $k - 1);}
    }
    return $co;
}
  
// driver program to test the above
// function
 
    $arr = array(1, 2, 3, 4, 5, 6);
    $k = 3;
    $d = 5;
    $n = sizeof($arr) / sizeof($arr[0]);
    echo find($arr, $n,$d,$k);
    return 0;
?>

Javascript




<script>
// Javascript code to find no. of subsets
// with maximum difference d between
// max and min of all K-size subsets
     
    // function to calculate factorial
    // of a number
    function fact(n)
    {
        let answer=1;
        if (n == 0 || n == 1)
           {    return answer;}
        else
        {
            for(var i = n; i >= 1; i--){
                  answer = answer * i;
            }
            return answer;
        }
    }
 
// function to count ways to select r
// numbers from n given numbers
function findcombination(n,r)
{
    return( Math.floor(fact(n) / (fact(n - r) *
                           fact(r))));
}
 
// function to return the total number
// of required subsets :
// n is the number of elements in array
// d is the maximum difference between
// minimum and maximum element in each
// subset of size k
function find(arr, n, d, k)
{
    arr.sort(function(a, b){return a-b;});
    let ans = 0, end = n, co = 0,
        start = 0;
   
    // loop to traverse from 0-n
    for (let i = 0; i < n; i++) {
   
    let val = arr[i] + d;
       
    // binary search to get the position
    // which will be stored in start
    start=i;
    while (start < end - 1){
        let mid = Math.floor((start + end) / 2);
   
        // if mid value greater than
        // arr[i]+d do search in
        // arr[start:mid]
        if (arr[mid] > val)
            end = mid;
        else
            start = mid+1;
        }
           
    if (start !=n && arr[start] <= val)
            start += 1;
   
        let c = start-i;
   
    // if the numbers of elements 'c' is
    // greater or equal to the given size k,
    // then only subsets of required size k
    // can be formed
    if (c >= k){
        co += findcombination(c - 1, k - 1);}
    }
       
    return co;
}
 
// driver program to test the above function
let arr = [1, 2, 3, 4, 5, 6];
let k = 3, d = 5;
let n = arr.length;
document.write(find(arr, n, d, k));
 
    // This code is contributed by rag2127.
</script>
Output
20

Output: 
 

20

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