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Maximum Bitwise AND value of subsequence of length K
• Difficulty Level : Easy
• Last Updated : 08 Jun, 2021

Given an array a of size N and an integer K. The task is to find the maximum bitwise and value of elements of any subsequence of length K
Note: a[i] <= 109
Examples:

Input: a[] = {10, 20, 15, 4, 14}, K = 4
Output:
{20, 15, 4, 14} is the subsequence with highest ‘&’ value.
Input: a[] = {255, 127, 31, 5, 24, 37, 15}, K = 5
Output: 8

Naive Approach: A naive approach is to recursively find the bitwise and value of all subsequences of length K and the maximum among all of them will be the answer.
Efficient Approach: An efficient approach is to solve it using bit properties. Below are the steps to solve the problem:

• Iterate from the left(initially left = 31 as 232 > 109 ) till we find > K numbers in the vector temp (initially temp = arr) whose i-th bit is set. Update the new set of numbers to temp array
• If we do not get > K numbers, the & value of any K elements in the temp array will be the maximum & value possible.
• Repeat Step-1 with left re-initialized as first-bit + 1.

Below is the implementation of the above approach:

## C++

 `// C++ program to find the sum of``// the addition of all possible subsets.``#include ``using` `namespace` `std;` `// Function to perform step-1``vector<``int``> findSubset(vector<``int``>& temp, ``int``& last, ``int` `k)``{``    ``vector<``int``> ans;` `    ``// Iterate from left till 0``    ``// till we get a bit set of K numbers``    ``for` `(``int` `i = last; i >= 0; i--) {``        ``int` `cnt = 0;` `        ``// Count the numbers whose``        ``// i-th bit is set``        ``for` `(``auto` `it : temp) {``            ``int` `bit = it & (1 << i);``            ``if` `(bit > 0)``                ``cnt++;``        ``}` `        ``// If the array has numbers>=k``        ``// whose i-th bit is set``        ``if` `(cnt >= k) {``            ``for` `(``auto` `it : temp) {``                ``int` `bit = it & (1 << i);``                ``if` `(bit > 0)``                    ``ans.push_back(it);``            ``}` `            ``// Update last``            ``last = i - 1;` `            ``// Return the new set of numbers``            ``return` `ans;``        ``}``    ``}` `    ``return` `ans;``}` `// Function to find the maximum '&' value``// of K elements in subsequence``int` `findMaxiumAnd(``int` `a[], ``int` `n, ``int` `k)``{``    ``int` `last = 31;``    ``// Temporary arrays``    ``vector<``int``> temp1, temp2;` `    ``// Initially temp = arr``    ``for` `(``int` `i = 0; i < n; i++) {``        ``temp2.push_back(a[i]);``    ``}` `    ``// Iterate till we have >=K elements``    ``while` `((``int``)temp2.size() >= k) {` `        ``// Temp array``        ``temp1 = temp2;` `        ``// Find new temp array if``        ``// K elements are there``        ``temp2 = findSubset(temp2, last, k);``    ``}` `    ``// Find the & value``    ``int` `ans = temp1;``    ``for` `(``int` `i = 0; i < k; i++)``        ``ans = ans & temp1[i];` `    ``return` `ans;``}` `// Driver Code``int` `main()``{``    ``int` `a[] = { 255, 127, 31, 5, 24, 37, 15 };``    ``int` `n = ``sizeof``(a) / ``sizeof``(a);``    ``int` `k = 4;` `    ``cout << findMaxiumAnd(a, n, k);``}`

## Java

 `// Java program to find the sum of``// the addition of all possible subsets.``import` `java.util.*;``class` `GFG``{``static` `int` `last;` `// Function to perform step-1``static` `Vector``       ``findSubset(Vector temp, ``int` `k)``{``    ``Vector ans = ``new` `Vector();` `    ``// Iterate from left till 0``    ``// till we get a bit set of K numbers``    ``for` `(``int` `i = last; i >= ``0``; i--)``    ``{``        ``int` `cnt = ``0``;` `        ``// Count the numbers whose``        ``// i-th bit is set``        ``for` `(Integer it : temp)``        ``{``            ``int` `bit = it & (``1` `<< i);``            ``if` `(bit > ``0``)``                ``cnt++;``        ``}` `        ``// If the array has numbers>=k``        ``// whose i-th bit is set``        ``if` `(cnt >= k)``        ``{``            ``for` `(Integer it : temp)``            ``{``                ``int` `bit = it & (``1` `<< i);``                ``if` `(bit > ``0``)``                    ``ans.add(it);``            ``}` `            ``// Update last``            ``last = i - ``1``;` `            ``// Return the new set of numbers``            ``return` `ans;``        ``}``    ``}``    ``return` `ans;``}` `// Function to find the maximum '&' value``// of K elements in subsequence``static` `int` `findMaxiumAnd(``int` `a[], ``int` `n, ``int` `k)``{``    ``last = ``31``;``    ` `    ``// Temporary arrays``    ``Vector temp1 = ``new` `Vector();``    ``Vector temp2 = ``new` `Vector();;` `    ``// Initially temp = arr``    ``for` `(``int` `i = ``0``; i < n; i++)``    ``{``        ``temp2.add(a[i]);``    ``}` `    ``// Iterate till we have >=K elements``    ``while` `((``int``)temp2.size() >= k)``    ``{` `        ``// Temp array``        ``temp1 = temp2;` `        ``// Find new temp array if``        ``// K elements are there``        ``temp2 = findSubset(temp2, k);``    ``}` `    ``// Find the & value``    ``int` `ans = temp1.get(``0``);``    ``for` `(``int` `i = ``0``; i < k; i++)``        ``ans = ans & temp1.get(i);` `    ``return` `ans;``}` `// Driver Code``public` `static` `void` `main(String[] args)``{``    ``int` `a[] = { ``255``, ``127``, ``31``, ``5``, ``24``, ``37``, ``15` `};``    ``int` `n = a.length;``    ``int` `k = ``4``;` `    ``System.out.println(findMaxiumAnd(a, n, k));``}``}` `// This code is contributed by 29AjayKumar`

## Python3

 `# Python3 program to find the sum of``# the addition of all possible subsets.``last ``=` `31` `# Function to perform step-1``def` `findSubset(temp, k):``    ``global` `last``    ``ans ``=` `[]` `    ``# Iterate from left till 0``    ``# till we get a bit set of K numbers``    ``for` `i ``in` `range``(last, ``-``1``, ``-``1``):``        ``cnt ``=` `0` `        ``# Count the numbers whose``        ``# i-th bit is set``        ``for` `it ``in` `temp:``            ``bit ``=` `it & (``1` `<< i)``            ``if` `(bit > ``0``):``                ``cnt ``+``=` `1` `        ``# If the array has numbers>=k``        ``# whose i-th bit is set``        ``if` `(cnt >``=` `k):``            ``for` `it ``in` `temp:``                ``bit ``=` `it & (``1` `<< i)``                ``if` `(bit > ``0``):``                    ``ans.append(it)` `            ``# Update last``            ``last ``=` `i ``-` `1` `            ``# Return the new set of numbers``            ``return` `ans` `    ``return` `ans` `# Function to find the maximum '&' value``# of K elements in subsequence``def` `findMaxiumAnd(a, n, k):``    ``global` `last` `    ``# Temporary arrays``    ``temp1, temp2, ``=` `[], []` `    ``# Initially temp = arr``    ``for` `i ``in` `range``(n):``        ``temp2.append(a[i])` `    ``# Iterate till we have >=K elements``    ``while` `len``(temp2) >``=` `k:` `        ``# Temp array``        ``temp1 ``=` `temp2` `        ``# Find new temp array if``        ``# K elements are there``        ``temp2 ``=` `findSubset(temp2, k)` `    ``# Find the & value``    ``ans ``=` `temp1[``0``]``    ``for` `i ``in` `range``(k):``        ``ans ``=` `ans & temp1[i]` `    ``return` `ans` `# Driver Code``a ``=` `[``255``, ``127``, ``31``, ``5``, ``24``, ``37``, ``15``]``n ``=` `len``(a)``k ``=` `4` `print``(findMaxiumAnd(a, n, k))` `# This code is contributed by Mohit Kumar`

## C#

 `// C# program to find the sum of``// the addition of all possible subsets.``using` `System;``using` `System.Collections.Generic;` `class` `GFG``{``static` `int` `last;` `// Function to perform step-1``static` `List<``int``>findSubset(List<``int``> temp, ``int` `k)``{``    ``List<``int``> ans = ``new` `List<``int``>();` `    ``// Iterate from left till 0``    ``// till we get a bit set of K numbers``    ``for` `(``int` `i = last; i >= 0; i--)``    ``{``        ``int` `cnt = 0;` `        ``// Count the numbers whose``        ``// i-th bit is set``        ``foreach` `(``int` `it ``in` `temp)``        ``{``            ``int` `bit = it & (1 << i);``            ``if` `(bit > 0)``                ``cnt++;``        ``}` `        ``// If the array has numbers>=k``        ``// whose i-th bit is set``        ``if` `(cnt >= k)``        ``{``            ``foreach` `(``int` `it ``in` `temp)``            ``{``                ``int` `bit = it & (1 << i);``                ``if` `(bit > 0)``                    ``ans.Add(it);``            ``}` `            ``// Update last``            ``last = i - 1;` `            ``// Return the new set of numbers``            ``return` `ans;``        ``}``    ``}``    ``return` `ans;``}` `// Function to find the maximum '&' value``// of K elements in subsequence``static` `int` `findMaxiumAnd(``int` `[]a, ``int` `n, ``int` `k)``{``    ``last = 31;``    ` `    ``// Temporary arrays``    ``List<``int``> temp1 = ``new` `List<``int``>();``    ``List<``int``> temp2 = ``new` `List<``int``>();;` `    ``// Initially temp = arr``    ``for` `(``int` `i = 0; i < n; i++)``    ``{``        ``temp2.Add(a[i]);``    ``}` `    ``// Iterate till we have >=K elements``    ``while` `((``int``)temp2.Count >= k)``    ``{` `        ``// Temp array``        ``temp1 = temp2;` `        ``// Find new temp array if``        ``// K elements are there``        ``temp2 = findSubset(temp2, k);``    ``}` `    ``// Find the & value``    ``int` `ans = temp1;``    ``for` `(``int` `i = 0; i < k; i++)``        ``ans = ans & temp1[i];` `    ``return` `ans;``}` `// Driver Code``public` `static` `void` `Main(String[] args)``{``    ``int` `[]a = { 255, 127, 31, 5, 24, 37, 15 };``    ``int` `n = a.Length;``    ``int` `k = 4;` `    ``Console.WriteLine(findMaxiumAnd(a, n, k));``}``}` `// This code is contributed by Rajput-Ji`

## Javascript

 ``
Output:
`24`

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