# Find a number M < N such that difference between their XOR and AND is maximum

• Last Updated : 28 Sep, 2021

Given a natural number N, the task is to find a number M smaller than N such that the difference between their bitwise XOR (N ^ M) and bitwise AND (N & M) is maximum.

Examples:

Input: N = 4
Output: 3
Explanation:
(4 ^ 0) – (4 & 0) = 4
(4 ^ 1) – (4 & 1) = 5
(4 ^ 2) – (4 & 2) = 6
(4 ^ 3) – (4 & 3) = 7
Hence, the value of M is 3.

Input: N = 6
Output: 1
Explanation:
The difference between N ^ M and N & M is maximum when M = 1.

Naive Approach: The idea is to iterate for every element less than N and find M for which N^M – N&M is maximum. Below are the steps:

• Initialize a variable let’s say maxDiff with 0 and M with -1.
• Iterate from 0 to N-1 and calculate diff = N^i -N&i.
• If diff is greater or equal to maxDiff assign M = i and maxDiff = diff.

Below is the implementation of the above approach:

## C++

 // C++ program for the above approach#include using namespace std; // Function to return M= maxDiff) {             // Update variables            maxDiff = diff;            M = i;        }    }     // Return the answer    return M;} // Driver Codeint main(){    // Given Number N    int N = 6;     // Function Call    cout << getMaxDifference(N);    return 0;}

## Java

 // Java program for the above approachclass GFG{ // Function to return M= maxDiff)        {             // Update variables            maxDiff = diff;            M = i;        }    }     // Return the answer    return M;} // Driver Codepublic static void main(String[] args){    // Given Number N    int N = 6;     // Function Call    System.out.print(getMaxDifference(N));}} // This code is contributed by Rohit_ranjan

## Python3

 # Python3 program for the above approach # Function to return M= maxDiff):                         # Update variables            maxDiff = diff;            M = i;     # Return the answer    return M; # Driver Codeif __name__ == '__main__':         # Given number N    N = 6;     # Function call    print(getMaxDifference(N)); # This code is contributed by amal kumar choubey

## C#

 // C# program for the above approachusing System; class GFG{ // Function to return M= maxDiff)        {                         // Update variables            maxDiff = diff;            M = i;        }    }     // Return the answer    return M;} // Driver Codepublic static void Main(String[] args){         // Given number N    int N = 6;     // Function call    Console.Write(getMaxDifference(N));}} // This code is contributed by Rajput-Ji

## Javascript



Output:

1

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

Efficient Approach: The idea is to observe that the difference between Bitwise XOR and Bitwise AND is maximum if Bitwise AND of the two numbers is the minimum possible number and the minimum possible number is 0
The Bitwise AND between the two numbers is zero if and only if they complement each other. Therefore, the possible value of M must be the complement of the given number N.
Below is the implementation of the above approach:

## C++

 // C++ program for the above approach#include using namespace std; // Function to flip all bits of Nint findM(int N){    int M = 0;     // Finding most significant bit of N    int MSB = (int)log2(N);     // Calculating required number    for (int i = 0; i < MSB; i++) {         if (!(N & (1 << i)))            M += (1 << i);    }     // Return the answer    return M;} // Driver Codeint main(){    // Given Number    int N = 6;     // Function Call    cout << findM(N);    return 0;}

## Java

 // Java program for the above approachclass GFG{ // Function to flip all bits of Nstatic int findM(int N){    int M = 0;     // Finding most significant bit of N    int MSB = (int)Math.log(N);     // Calculating required number    for(int i = 0; i < MSB; i++)    {        if ((N & (1 << i)) == 0)            M += (1 << i);    }     // Return the answer    return M;} // Driver Codepublic static void main(String[] args){         // Given number    int N = 6;     // Function call    System.out.print(findM(N));}} // This code is contributed by Rajput-Ji

## Python3

 # Python3 program for the above approachimport math # Function to flip all bits of Ndef findM(N):         M = 0;     # Finding most significant bit of N    MSB = int(math.log(N));     # Calculating required number    for i in range(MSB):        if ((N & (1 << i)) == 0):            M += (1 << i);         # Return the answer    return M; # Driver Codeif __name__ == '__main__':     # Given number    N = 6;     # Function call    print(findM(N)); # This code is contributed by Amit Katiyar

## C#

 // C# program for the above approachusing System; class GFG{ // Function to flip all bits of Nstatic int findM(int N){    int M = 0;     // Finding most significant bit of N    int MSB = (int)Math.Log(N);     // Calculating required number    for(int i = 0; i < MSB; i++)    {        if ((N & (1 << i)) == 0)            M += (1 << i);    }     // Return the answer    return M;} // Driver Codepublic static void Main(String[] args){         // Given number    int N = 6;     // Function call    Console.Write(findM(N));}} // This code is contributed by Amit Katiyar

## Javascript



Output:

1

Time Complexity: O(log2N)
Auxiliary Space: O(1)

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