# Count of pairs {X, Y} from an array such that sum of count of set bits in X ⊕ Y and twice the count of set bits in X & Y is M

• Difficulty Level : Expert
• Last Updated : 07 Aug, 2022

Given an array arr[] consisting of N non-negative integers and an integer M, the task is to find the count of unordered pairs {X, Y} of array elements that satisfies the condition setBits(X ⊕ Y) + 2 * setBits(X & Y) = M, where denotes the Bitwise XOR and & denotes the Bitwise AND.

Examples:

Input: arr[] = {30, 0, 4, 5 }, M = 2
Output: 2
Explanation: The pairs satisfying the necessary conditions are {{3, 0}, {0, 5}}.

Input: arr[] = {1, 2, 3, 4}, M = 3
Output: 3
Explanation: The pairs satisfying the necessary conditions are {{1, 3}, {2, 3}, {3, 4}}.

Naive Approach: The simplest approach is to generate all possible pairs from the given array and for each pair, check if the necessary condition is satisfied or not. Increment the count for pairs satisfying the given conditions and finally, print the count of pairs.

Time Complexity: O(N2)
Auxiliary Space: O(1)

Efficient Approach: The above approach can be optimized based on the following observations:

• From the property of Bitwise XOR:
• setBits( a⊕ b ) = setBits( a|b ) – setBits( a&b )
• setBits( a|b ) = setBits(a) + setBits(b) – setBits( a&b )
• Therefore, the given equation becomes:
• ( setBits( X|Y ) – setBits( X&Y ) )+( 2 × setBits( X&Y ) ) = M
• setBits( X ) + setBits ( Y ) – setBits( X&Y ) – setBits( X&Y ) + ( 2 × setBits ( X&Y ) ) = M
• setBits( X ) + setBits( Y ) = M
• Therefore, the task reduces to counting the pairs of elements whose sum of set bits is equal to M.

Follow the steps below to solve the problem:

Below is the implementation of the above approach:

## C++

 `// C++ program for the above approach` `#include ` `using` `namespace` `std;`   `// Function to count number` `// of setbits in the number n` `int` `countsetbits(``int` `n)` `{` `    ``// Stores the count of setbits` `    ``int` `count = 0;`   `    ``// Iterate while N is` `    ``// greater than 0` `    ``while` `(n) {`   `        ``// Increment count by 1` `        ``// if N is odd` `        ``count += n & 1;`   `        ``// Right shift N` `        ``n >>= 1;` `    ``}`   `    ``// Return the count of set bits` `    ``return` `(count);` `}`   `// Function to find total count of` `// given pairs satisfying the equation` `int` `countPairs(``int` `a[], ``int` `N, ``int` `M)` `{`   `    ``for` `(``int` `i = 0; i < N; i++) {`   `        ``// Update arr[i] with the count` `        ``// of set bits of arr[i]` `        ``a[i] = countsetbits(a[i]);` `    ``}`   `    ``// Stores the frequency` `    ``// of each array element` `    ``unordered_map<``int``, ``int``> mp;`   `    ``// Traverse the array` `    ``for` `(``int` `i = 0; i < N; i++) {`   `        ``// Increment the count` `        ``// of arr[i] in mp` `        ``mp[a[i]]++;` `    ``}`   `    ``// Stores the total count of pairs` `    ``int` `count = 0;`   `    ``// Traverse the array arr[]` `    ``for` `(``int` `i = 0; i < N; i++) {`   `        ``// Increment count by mp[M - a[i]]` `        ``count += mp[M - a[i]];`   `        ``// If a[i] is equal to M-a[i]` `        ``if` `(a[i] == M - a[i]) {`   `            ``// Decrement count by 1` `            ``count--;` `        ``}` `    ``}`   `    ``// Return count/2` `    ``return` `(count / 2);` `}`   `// Driver Code` `int` `main()` `{` `    ``// Input` `    ``int` `arr[] = { 3, 0, 4, 5 };` `    ``int` `N = ``sizeof``(arr) / ``sizeof``(arr);` `    ``int` `M = 2;`   `    ``cout << countPairs(arr, N, M);` `}`

## Java

 `// Java program for the above approach` `import` `java.io.*;` `import` `java.util.*;`   `class` `GFG{`   `// Function to count number` `// of setbits in the number n` `static` `int` `countsetbits(``int` `n)` `{` `    `  `    ``// Stores the count of setbits` `    ``int` `count = ``0``;`   `    ``// Iterate while N is` `    ``// greater than 0` `    ``while` `(n != ``0``) ` `    ``{` `        `  `        ``// Increment count by 1` `        ``// if N is odd` `        ``count += n & ``1``;`   `        ``// Right shift N` `        ``n >>= ``1``;` `    ``}`   `    ``// Return the count of set bits` `    ``return` `(count);` `}`   `// Function to find total count of` `// given pairs satisfying the equation` `static` `int` `countPairs(``int``[] a, ``int` `N, ``int` `M)` `{` `    ``for``(``int` `i = ``0``; i < N; i++)` `    ``{` `        `  `        ``// Update arr[i] with the count` `        ``// of set bits of arr[i]` `        ``a[i] = countsetbits(a[i]);` `    ``}`   `    ``// Stores the frequency` `    ``// of each array element` `     ``HashMap mp = ``new` `HashMap();`   `    ``// Traverse the array` `    ``for` `(``int` `i = ``0``; i < N; i++)` `    ``{` `        ``if` `(mp.containsKey(a[i]))` `        ``{` `            ``mp.put(a[i], mp.get(a[i]) + ``1``);` `        ``}` `        ``else` `        ``{` `            ``mp.put(a[i], ``1``);` `        ``}` `    ``}` `    `  `    ``// Stores the total count of pairs` `    ``int` `count = ``0``;`   `    ``// Traverse the array arr[]` `    ``for``(``int` `i = ``0``; i < N; i++) ` `    ``{` `        `  `        ``// Increment count by mp[M - a[i]]` `        ``count += mp.get(M - a[i]);`   `        ``// If a[i] is equal to M-a[i]` `        ``if` `(a[i] == M - a[i]) ` `        ``{` `            `  `            ``// Decrement count by 1` `            ``count--;` `        ``}` `    ``}`   `    ``// Return count/2` `    ``return` `(count / ``2``);` `}    `   `// Driver Code` `public` `static` `void` `main(String[] args)` `{` `    `  `    ``// Input` `    ``int``[] arr = { ``3``, ``0``, ``4``, ``5` `};` `    ``int` `N = arr.length;` `    ``int` `M = ``2``;`   `    ``System.out.println(countPairs(arr, N, M));` `}` `}`   `// This code is contributed by avijitmondal1998`

## Python3

 `# Python3 Program for the above approach`   `# Function to count number` `# of setbits in the number n` `def`  `countSetBits(n): ` `  `  `    ``# Stores the count of setbits` `    ``count ``=` `0` `    `  `    ``# Iterate while N is` `    ``# greater than 0` `    ``while` `(n): ` `      `  `        ``# Increment count by 1` `        ``# if N is odd` `        ``count ``+``=` `n & ``1` `        `  `        ``# Right shift N` `        ``n >>``=` `1` `        `  `    ``# Return the count of set bits` `    ``return` `count`   `def` `countPairs(arr, N, M):` `    ``for` `i ``in` `range``(``0``, N):` `      `  `        ``# Update arr[i] with the count` `        ``# of set bits of arr[i]` `        ``arr[i] ``=` `countSetBits(arr[i])` `        `  `    ``# Store counts of all elements in a dictionary` `    ``mp ``=` `{}` `    ``for` `i ``in` `range``(``0``, N):` `        ``if` `arr[i] ``in` `mp:` `            ``mp[arr[i]] ``+``=` `1` `        ``else``:` `            ``mp[arr[i]] ``=` `1` `    ``twice_count ``=` `0` `    `  `    ``# Iterate through each element and increment` `    ``# the count (Notice that every pair is` `    ``# counted twice)` `    ``for` `i ``in` `range``(``0``, N):` `        ``if` `M ``-` `arr[i] ``in` `mp.keys():` `            ``twice_count ``+``=` `mp[M ``-` `arr[i]]` `            `  `        ``# if (arr[i], arr[i]) pair satisfies the` `        ``# condition, then we need to ensure that` `        ``# the count is  decreased by one such` `        ``# that the (arr[i], arr[i]) pair is not` `        ``# considered` `        ``if` `(M ``-` `arr[i] ``=``=` `arr[i]):` `            ``twice_count ``-``=` `1` ` `  `    ``# return the half of twice_count` `    ``return` `int``(twice_count ``/` `2``)` `        `  `# Driver code` `# Input` `arr ``=` `[ ``3``, ``0``, ``4``, ``5``]` `N ``=` `len``(arr)` `M ``=` `2` `print``(countPairs(arr, N, M))`   `# This code is contributed by santhoshcharan.`

## C#

 `// C# program for the above approach` `using` `System;` `using` `System.Collections.Generic;`   `class` `GFG{`   `// Function to count number` `// of setbits in the number n` `static` `int` `countsetbits(``int` `n)` `{` `    `  `    ``// Stores the count of setbits` `    ``int` `count = 0;`   `    ``// Iterate while N is` `    ``// greater than 0` `    ``while` `(n != 0) ` `    ``{` `        `  `        ``// Increment count by 1` `        ``// if N is odd` `        ``count += n & 1;`   `        ``// Right shift N` `        ``n >>= 1;` `    ``}`   `    ``// Return the count of set bits` `    ``return` `(count);` `}`   `// Function to find total count of` `// given pairs satisfying the equation` `static` `int` `countPairs(``int``[] a, ``int` `N, ``int` `M)` `{` `    ``for``(``int` `i = 0; i < N; i++)` `    ``{` `        `  `        ``// Update arr[i] with the count` `        ``// of set bits of arr[i]` `        ``a[i] = countsetbits(a[i]);` `    ``}`   `    ``// Stores the frequency` `    ``// of each array element` `    ``Dictionary<``int``, ` `               ``int``> mp = ``new` `Dictionary<``int``, ` `                                        ``int``>();`   `    ``// Traverse the array` `    ``for``(``int` `i = 0; i < N; ++i) ` `    ``{` `           `  `        ``// Update frequency of` `        ``// each array element` `        ``if` `(mp.ContainsKey(a[i]) == ``true``)` `            ``mp[a[i]] += 1;` `        ``else` `            ``mp[a[i]] = 1;` `    ``}` `    `  `    ``// Stores the total count of pairs` `    ``int` `count = 0;`   `    ``// Traverse the array arr[]` `    ``for``(``int` `i = 0; i < N; i++) ` `    ``{` `        `  `        ``// Increment count by mp[M - a[i]]` `        ``count += mp[M - a[i]];`   `        ``// If a[i] is equal to M-a[i]` `        ``if` `(a[i] == M - a[i]) ` `        ``{` `            `  `            ``// Decrement count by 1` `            ``count--;` `        ``}` `    ``}`   `    ``// Return count/2` `    ``return` `(count / 2);` `}`   `// Driver Code` `static` `public` `void` `Main()` `{` `    `  `    ``// Input` `    ``int``[] arr = { 3, 0, 4, 5 };` `    ``int` `N = arr.Length;` `    ``int` `M = 2;`   `    ``Console.WriteLine(countPairs(arr, N, M));` `}` `}`   `// This code is contributed by sanjoy_62`

## Javascript

 ``

Output:

`2`

Time Complexity: O(NlogN)
Auxiliary Space: O(N) as using auxiliary space for unordered_map

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