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Sum of subsets of all the subsets of an array | O(N)

 3 years ago
source link: https://www.geeksforgeeks.org/sum-of-subsets-of-all-the-subsets-of-an-array-on/
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Sum of subsets of all the subsets of an array
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Sum of subsets of all the subsets of an array | O(N)
  • Difficulty Level : Hard
  • Last Updated : 06 Nov, 2019

Given an array arr[] of length N, the task is to find the overall sum of subsets of all the subsets of the array.

Examples:

Input: arr[] = {1, 1}
Output: 6
All possible subsets:
a) {} : 0
All the possible subsets of this subset
will be {}, Sum = 0
b) {1} : 1
All the possible subsets of this subset
will be {} and {1}, Sum = 0 + 1 = 1
c) {1} : 1
All the possible subsets of this subset
will be {} and {1}, Sum = 0 + 1 = 1
d) {1, 1} : 4
All the possible subsets of this subset
will be {}, {1}, {1} and {1, 1}, Sum = 0 + 1 + 1 + 2 = 4
Thus, ans = 0 + 1 + 1 + 4 = 6

Input: arr[] = {1, 4, 2, 12}
Output: 513

Approach: In this article, an approach with O(N) time complexity to solve the given problem will be discussed.
The key is observing the number of times an element will repeat in all the subsets.

Let’s magnify the view. It is known that every element will appear 2(N – 1) times in the sum of subsets. Now, let’s magnify the view even further and see how the count varies with the subset size.

There are N – 1CK – 1 subsets of size K for every index that include it.
Contribution of an element for a subset of size K will be equal to 2(K – 1) times its value. Thus, total contribution of an element for all the subsets of length K will be equal to N – 1CK – 1 * 2(K – 1)
Total contribution among all the subsets will be equal to:

N – 1CN – 1 * 2(N – 1) + N – 1CN – 2 * 2(N – 2 + N – 1CN – 3 * 2(N – 3) + … + N – 1C0 * 20.

Now, the contribution of each element in the final answer is known. So, multiply it to the sum of all the elements of the array which will give the required answer.

Below is the implementation of the above approach:

  • Python3

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// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
#define maxN 10
// To store factorial values
int fact[maxN];
// Function to return ncr
int ncr(int n, int r)
{
return (fact[n] / fact[r]) / fact[n - r];
}
// Function to return the required sum
int findSum(int* arr, int n)
{
// Intialising factorial
fact[0] = 1;
for (int i = 1; i < n; i++)
fact[i] = i * fact[i - 1];
// Multiplier
int mul = 0;
// Finding the value of multipler
// according to the formula
for (int i = 0; i <= n - 1; i++)
mul += (int)pow(2, i) * ncr(n - 1, i);
// To store the final answer
int ans = 0;
// Calculate the final answer
for (int i = 0; i < n; i++)
ans += mul * arr[i];
return ans;
}
// Driver code
int main()
{
int arr[] = { 1, 1 };
int n = sizeof(arr) / sizeof(int);
cout << findSum(arr, n);
return 0;
}
Output: 
6

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