# Count of elements which cannot form any pair whose sum is power of 2

Given an array arr[] of length N, the task is to print the number of array elements which cannot form a pair with any other array element whose sum is a power of two.

Examples:

Input: arr[] = {6, 2, 11}
Output: 1
Explanation:
Since 6 and 2 can form a pair with sum 8 (= 23). So only 11 has to be removed as it does not form a sum which is a power of 2.

Input: arr[] = [1, 1, 1, 1023],
Output: 0
Explanation:
The given array elements can be split into following two pairs:
(1, 1) with sum 2(= 21)
(1, 1023) with sum 1024(= 210)
Hence, no need to remove any element.

Approach:
To solve the problem mentioned above, follow the steps below:

• Store the frequencies of all array elements in a Map.
• For every array element a[i], iterate over all possible sums p = {20, 21, …., 230} and check if p – a[i] is present in the array or not.
• Either of the following two conditions need to be satisfied:
1. Let s = p – a[i]. If s is present in the array more than once, then a pair consisting of a[i] with sum p is possible.
2. If s is present only once in the array, then s has to be different from a[i] for a possible pair.
3. If none of the above two conditions are satisfied, no pair consisting of a[i] is possible with sum p.
• If the above two conditions does not satisfy for any p for the current a[i], then increase count as a[i] cannot form a sum which is a power of 2 with any other array element.
• Print the final value of count.

Below is the implementation of the above approach:

## C++

 `// C++ Program to count of ` `// array elements which do ` `// not form a pair with sum ` `// equal to a power of 2 ` `// with any other array element ` ` `  `#include ` `using` `namespace` `std; ` ` `  `// Function to calculate ` `// and return the ` `// count of elements ` ` `  `int` `powerOfTwo(``int` `a[], ``int` `n) ` `{ ` `    ``// Stores the frequencies ` `    ``// of every array element ` ` `  `    ``map<``int``, ``int``> mp; ` ` `  `    ``for` `(``int` `i = 0; i < n; i++) ` `        ``mp[a[i]]++; ` ` `  `    ``// Stores the count ` `    ``// of removals ` ` `  `    ``int` `count = 0; ` ` `  `    ``for` `(``int` `i = 0; i < n; i++) { ` `        ``bool` `f = ``false``; ` ` `  `        ``// For every element, check if ` `        ``// it can form a sum equal to ` `        ``// any power of 2 with any other ` `        ``// element ` ` `  `        ``for` `(``int` `j = 0; j < 31; j++) { ` ` `  `            ``// Store pow(2, j) - a[i] ` `            ``int` `s = (1 << j) - a[i]; ` ` `  `            ``// Check if s is present ` `            ``// in the array ` `            ``if` `(mp.count(s) ` ` `  `                ``// If frequency of s ` `                ``// exceeds 1 ` `                ``&& (mp[s] > 1 ` ` `  `                    ``// If s has frequency 1 ` `                    ``// but is different from ` `                    ``// a[i] ` `                    ``|| mp[s] == 1 && s != a[i])) ` ` `  `                ``// Pair possible ` `                ``f = ``true``; ` `        ``} ` ` `  `        ``// If no pair possible for ` `        ``// the current element ` ` `  `        ``if` `(f == ``false``) ` `            ``count++; ` `    ``} ` ` `  `    ``// Return the answer ` `    ``return` `count; ` `} ` ` `  `// Driver Code ` `int` `main() ` `{ ` `    ``int` `a[] = { 6, 2, 11 }; ` `    ``int` `n = ``sizeof``(a) / ``sizeof``(a); ` `    ``cout << powerOfTwo(a, n); ` ` `  `    ``return` `0; ` `} `

Output:

```1
```

Time Complexity: O(N)
Auxiliary Space: O(N)

Don’t stop now and take your learning to the next level. Learn all the important concepts of Data Structures and Algorithms with the help of the most trusted course: DSA Self Paced. Become industry ready at a student-friendly price.

My Personal Notes arrow_drop_up Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.

Article Tags :
Practice Tags :

1

Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.