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Minimum number of elements that should be removed to make the array good
• Last Updated : 25 Apr, 2019

Given an array arr[], the task is to find the minimum number of elements that must be removed to make the array good. A sequence a1, a2 … an is called good if for each element ai, there exists an element aj (i not equals to j) such that ai + aj is a power of two i.e. 2d for some non-negative integer d.

Examples:

Input: arr[] = {4, 7, 1, 5, 4, 9}
Output: 1
Remove 5 from the array to make the array good.

Input: arr[] = {1, 3, 1, 1}
Output: 0

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach: We should delete only such ai for which there is no aj (i not equals to j) such that ai + aj is a power of 2.
For each value let’s find the number of its occurrences in the array. We can use the map data-structure.

Now we can easily check that ai doesn’t have a pair aj. Let’s iterate over all possible sums, S = 20, 21, …, 230 and for each S calculate S – a[i] whether it exists in the map.

Below is the implementation of the above approach :

## C++

 `// C++ implementation of the approach``#include ``using` `namespace` `std;`` ` `// Function to return the minimum number``// of elements that must be removed``// to make the given array good``int` `minimumRemoval(``int` `n, ``int` `a[])``{`` ` `    ``map<``int``, ``int``> c;`` ` `    ``// Count frequency of each element``    ``for` `(``int` `i = 0; i < n; i++)``        ``c[a[i]]++;`` ` `    ``int` `ans = 0;`` ` `    ``// For each element check if there``    ``// exists another element that makes``    ``// a valid pair``    ``for` `(``int` `i = 0; i < n; i++) {``        ``bool` `ok = ``false``;``        ``for` `(``int` `j = 0; j < 31; j++) {``            ``int` `x = (1 << j) - a[i];``            ``if` `(c.count(x) && (c[x] > 1``                       ``|| (c[x] == 1 && x != a[i]))) {``                ``ok = ``true``;``                ``break``;``            ``}``        ``}`` ` `        ``// If does not exist then``        ``// increment answer``        ``if` `(!ok)``            ``ans++;``    ``}`` ` `    ``return` `ans;``}`` ` `// Driver code``int` `main()``{``    ``int` `a[] = { 4, 7, 1, 5, 4, 9 };``    ``int` `n = ``sizeof``(a) / ``sizeof``(a);``    ``cout << minimumRemoval(n, a);`` ` `    ``return` `0;``}`

## Java

 `// Java implementation of the approach``import` `java.util.*;`` ` `class` `GFG ``{`` ` `// Function to return the minimum number``// of elements that must be removed``// to make the given array good``static` `int` `minimumRemoval(``int` `n, ``int` `a[])``{`` ` `    ``Map c = ``new` `HashMap<>();`` ` `    ``// Count frequency of each element``    ``for` `(``int` `i = ``0``; i < n; i++)``        ``if``(c.containsKey(a[i]))``        ``{``            ``c.put(a[i], c.get(a[i])+``1``);``        ``}``        ``else``        ``{``            ``c.put(a[i], ``1``);``        ``}`` ` `    ``int` `ans = ``0``;`` ` `    ``// For each element check if there``    ``// exists another element that makes``    ``// a valid pair``    ``for` `(``int` `i = ``0``; i < n; i++) ``    ``{``        ``boolean` `ok = ``false``;``        ``for` `(``int` `j = ``0``; j < ``31``; j++)``        ``{``            ``int` `x = (``1` `<< j) - a[i];``            ``if` `((c.get(x) != ``null` `&& (c.get(x) > ``1``)) ||``                ``c.get(x) != ``null` `&& (c.get(x) == ``1` `&& x != a[i])) ``            ``{``                ``ok = ``true``;``                ``break``;``            ``}``        ``}`` ` `        ``// If does not exist then``        ``// increment answer``        ``if` `(!ok)``            ``ans++;``    ``}`` ` `    ``return` `ans;``}`` ` `// Driver code``public` `static` `void` `main(String[] args)``{``    ``int` `a[] = { ``4``, ``7``, ``1``, ``5``, ``4``, ``9` `};``    ``int` `n = a.length;``    ``System.out.println(minimumRemoval(n, a));``}``}`` ` `/* This code contributed by PrinciRaj1992 */`

## Python3

 `# Python3 implementation of the approach `` ` `# Function to return the minimum number ``# of elements that must be removed ``# to make the given array good ``def` `minimumRemoval(n, a) :`` ` `    ``c ``=` `dict``.fromkeys(a, ``0``);`` ` `    ``# Count frequency of each element ``    ``for` `i ``in` `range``(n) :``        ``c[a[i]] ``+``=` `1``; `` ` `    ``ans ``=` `0``; `` ` `    ``# For each element check if there ``    ``# exists another element that makes ``    ``# a valid pair ``    ``for` `i ``in` `range``(n) :``        ``ok ``=` `False``; ``        ``for` `j ``in` `range``(``31``) :``             ` `            ``x ``=` `(``1` `<< j) ``-` `a[i]; ``            ``if` `(x ``in` `c ``and` `(c[x] > ``1` `or` `               ``(c[x] ``=``=` `1` `and` `x !``=` `a[i]))) :``                 ` `                ``ok ``=` `True``; ``                ``break``;`` ` `        ``# If does not exist then ``        ``# increment answer ``        ``if` `(``not` `ok) :``            ``ans ``+``=` `1``; ``             ` `    ``return` `ans; `` ` `# Driver Code``if` `__name__ ``=``=` `"__main__"` `:``     ` `    ``a ``=` `[ ``4``, ``7``, ``1``, ``5``, ``4``, ``9` `]; ``    ``n ``=` `len``(a) ; ``     ` `    ``print``(minimumRemoval(n, a));``     ` `# This code is contributed by Ryuga`

## C#

 `// C# implementation of the approach``using` `System.Linq;``using` `System;`` ` `class` `GFG``{``// Function to return the minimum number``// of elements that must be removed``// to make the given array good``static` `int` `minimumRemoval(``int` `n, ``int` `[]a)``{`` ` `    ``int``[] c = ``new` `int``;`` ` `    ``// Count frequency of each element``    ``for` `(``int` `i = 0; i < n; i++)``        ``c[a[i]]++;`` ` `    ``int` `ans = 0;`` ` `    ``// For each element check if there``    ``// exists another element that makes``    ``// a valid pair``    ``for` `(``int` `i = 0; i < n; i++) ``    ``{``        ``bool` `ok = ``true``;``        ``for` `(``int` `j = 0; j < 31; j++) ``        ``{``            ``int` `x = (1 << j) - a[i];``            ``if` `(c.Contains(x) && (c[x] > 1 ||``                    ``(c[x] == 1 && x != a[i]))) ``            ``{``                ``ok = ``false``;``                ``break``;``            ``}``        ``}`` ` `        ``// If does not exist then``        ``// increment answer``        ``if` `(!ok)``            ``ans++;``    ``}`` ` `    ``return` `ans;``}`` ` `// Driver code``static` `void` `Main()``{``    ``int` `[]a = { 4, 7, 1, 5, 4, 9 };``    ``int` `n = a.Length;``    ``Console.WriteLine(minimumRemoval(n, a));``}``}`` ` `// This code is contributed by mits`
Output:
```1
```

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