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Program to invert bits of a number Efficiently

  • Difficulty Level : Medium
  • Last Updated : 28 Apr, 2021

Given a non-negative integer N. The task is to invert the bits of the number N and print the decimal equivalent of the number obtained after inverting the bits. 
Note: Leading 0’s are not being considered.
Examples: 
 

Input : 11
Output : 4
(11)10 = (1011)2
After inverting the bits, we get:
(0100)2 = (4)10.

Input : 20
Output : 11
(20)10 = (10100)2.
After inverting the bits, we get:
(01011)2 = (11)10.

 

A similar problem is already discussed in Invert actual bits of a number.
In this article, an efficient approach using bitwise operators is discussed. Below is the step by step algorithm to solve the problem:
 

  1. Calculate the total number of bits in the given number. This can be done by calculating:
    X = log2N

    Where N is the given number and X is the total number of bits of N.

  2. The next step is to generate a number with X bits and all bits set. That is, 11111….X-times. This can be done by calculating:
    Step-1: M = 1 << X
    Step-2: M = M | (M-1)

    Where M is the required X-bit number with all bits set.

  3. The final step is to calculate the bit-wise XOR of M with N, which will be our answer.

Below is the implementation of the above approach: 
 



C++




// C++ program to invert actual bits
// of a number.
#include <bits/stdc++.h>
  
using namespace std;
  
// Function to invert bits of a number
int invertBits(int n)
{
    // Calculate number of bits of N-1;
    int x = log2(n) ;
  
    int m = 1 << x;
  
    m = m | m - 1;
  
    n = n ^ m;
  
    return n;
}
  
// Driver code
int main()
{
    int n = 20;
  
    cout << invertBits(n);
  
    return 0;
}

Java




// Java program to invert 
// actual bits of a number. 
import java.util.*;
  
class GFG
{
// Function to invert 
// bits of a number 
static int invertBits(int n) 
    // Calculate number of bits of N-1; 
    int x = (int)(Math.log(n) / 
                  Math.log(2)) ; 
  
    int m = 1 << x; 
  
    m = m | m - 1
  
    n = n ^ m; 
  
    return n; 
  
// Driver code 
public static void main(String[] args) 
    int n = 20
  
    System.out.print(invertBits(n)); 
}
  
// This code is contributed by Smitha

Python3




# Python3 program to invert actual
# bits of a number.
import math
  
# Function to invert bits of a number
def invertBits(n):
      
    # Calculate number of bits of N-1
    x = int(math.log(n, 2))
  
    m = 1 << x
  
    m = m | m - 1
  
    n = n ^ m
  
    return n
  
# Driver code
n = 20
  
print(invertBits(n))
  
# This code is contributed 29AjayKumar

C#




// C# program to invert 
// actual bits of a number. 
using System;
  
  
   
public class GFG
{
// Function to invert 
// bits of a number 
static int invertBits(int n) 
    // Calculate number of bits of N-1; 
    int x = (int)(Math.Log(n) / 
                  Math.Log(2)) ; 
   
    int m = 1 << x; 
   
    m = m | m - 1; 
   
    n = n ^ m; 
   
    return n; 
   
// Driver code 
public static void Main() 
    int n = 20; 
   
    Console.Write(invertBits(n)); 
}
   
// This code is contributed by Subhadeep

PHP




<?php
// PHP program to invert actual 
// bits of a number.
  
// Function to invert bits 
// of a number
function invertBits($n)
{
    // Calculate number of
    // bits of N-1;
    $x = log($n, 2);
  
    $m = 1 << $x;
  
    $m = $m | $m - 1;
  
    $n = $n ^ $m;
  
    return $n;
}
  
// Driver code
$n = 20;
  
echo(invertBits($n));
  
// This code is contributed 
// by mits
?>

Javascript




<script>
  
// Javascript program to invert actual bits
// of a number.
  
// Function to invert bits of a number
function invertBits(n)
{
    // Calculate number of bits of N-1;
    let x = parseInt(Math.log(n) / Math.log(2)) ;
  
    let m = 1 << x;
  
    m = m | m - 1;
  
    n = n ^ m;
  
    return n;
}
  
// Driver code
    let n = 20;
  
    document.write(invertBits(n));
  
</script>
Output: 
11

 

Time Complexity: O(log2n) 
Auxiliary Space: O(1)
 

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