Find One's Complement of an Integer

Last Updated : 30 Jun, 2026

Given an integer n, find the one's complement of the integer. 
Examples: 

Input : n = 5
Output : 2

Input : n = 255
Output : 0

Input : n = 26
Output : 5

Try It Yourself
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Table of Content

[Naive Approach] Binary Conversion with Flip - O(log n) Time and O(log n) Space

Convert the number to binary, flip all bits (0 to1, 1 to 0), then convert back to decimal.

  • Convert n to binary and store in array
  • Reverse array to get correct binary order
  • Flip each bit: change 0 to 1 and 1 to 0
  • Convert flipped binary back to decimal
C++ 
// CPP program to find 1's complement of n.
#include <bits/stdc++.h>
using namespace std;

int onesComplement(int n)
{
    vector<int> v;
    
    // convert to binary representation
    while (n != 0) {
        v.push_back(n % 2);
        n = n / 2;
    }
    
    reverse(v.begin(), v.end());
    
    // change 1's to 0 and 0's to 1
    for (int i = 0; i < v.size(); i++) {
        if (v[i] == 0)
            v[i] = 1;
        else
            v[i] = 0;
    }
    
    // convert back to number representation
    int two = 1;
    for (int i = v.size() - 1; i >= 0; i--) {
        n = n + v[i] * two;
        two = two * 2;
    }
    return n;
}

int main()
{
    int n = 22;
    cout << onesComplement(n);
    return 0;
}
Java 
// Java program to find 1's complement of n.

import java.util.*;

class GFG {
    static int onesComplement(int n)
    {
        ArrayList<Integer> v = new ArrayList<Integer>();
        
        // convert to binary representation
        while (n != 0) {
            v.add(n % 2);
            n = n / 2;
        }

        Collections.reverse(v);

        // change 1's to 0 and 0's to 1
        for (int i = 0; i < v.size(); i++) {
            if (v.get(i) == 0)
                v.set(i, 1);
            else
                v.set(i, 0);
        }
        
        // convert back to number representation
        int two = 1;
        for (int i = v.size() - 1; i >= 0; i--) {
            n = n + v.get(i) * two;
            two = two * 2;
        }
        return n;
    }

    // Driver code
    public static void main(String[] args)
    {
        int n = 22;
        System.out.println(onesComplement(n));
    }
}
Python 
# Python3 program to find 1's complement of n.
def onesComplement(n):
    v = []
    
    # convert to binary representation
    while (n != 0):
        v.append(n % 2)
        n = n // 2

    v.reverse()

    # change 1's to 0 and 0's to 1
    for i in range(len(v)):
        if (v[i] == 0):
            v[i] = 1
        else:
            v[i] = 0

    # convert back to number representation
    two = 1
    for i in range(len(v) - 1, -1, -1):
        n = n + v[i] * two
        two = two * 2

    return n

# Driver code
n = 22
print(onesComplement(n))
C# 
// C# program to find 1's complement of n.
using System;
using System.Collections;

public class GFG{
    
    public static int onesComplement(int n)
    {
        ArrayList v = new ArrayList();
        // convert to binary representation
        while (n != 0) {
            v.Add(n % 2);
            n = n / 2;
        }

        v.Reverse();

        // change 1's to 0 and 0's to 1
        for (int i = 0; i < v.Count; i++) {
            if (Convert.ToInt32(v[i]) == 0)
                v[i]= 1;
            else
                v[i]= 0;
        }
        
        // convert back to number representation
        int two = 1;
        for (int i = v.Count - 1; i >= 0; i--) {
            n = n + Convert.ToInt32(v[i]) * two;
            two = two * 2;
        }
        return n;
    }
    
    // Driver code
    static public void Main (){
        int n = 22;
        Console.WriteLine(onesComplement(n));
    }
}
JavaScript 
// JavaScript program to find 1's complement of n.
function onesComplement(n)
{
    let v = [];
    
    // convert to binary representation
    while (n != 0) {
        v.push(n % 2);
        n = Math.floor(n / 2);
    }
    v.reverse();
    
    // change 1's to 0 and 0's to 1
    for (var i = 0; i < v.length; i++) {
        if (v[i] == 0)
            v[i] = 1;
        else
            v[i] = 0;
    }
    
    // convert back to number representation
    let two = 1;
    for (let i = v.length - 1; i >= 0; i--) {
        n = n + v[i] * two;
        two = two * 2;
    }
    return n;
}

// Driver code
let n = 22;
console.log(onesComplement(n));

Output
9

[Expected Approach] Bit Manipulation - O(1) Time and O(1) Space 

Find the number of bits required to represent n. Create a mask with all bits set up to that length. XOR n with mask to flip all bits.

  • Calculate number_of_bits = floor(log2(n)) + 1
  • Create mask = (1 << number_of_bits) - 1
  • Return mask XOR n
C++ 
// CPP program to find 1's complement of n.
#include <bits/stdc++.h>
using namespace std;

unsigned int onesComplement(unsigned int n)
{
    // Find number of bits in the given integer
    int number_of_bits = floor(log2(n)) + 1;

    // XOR the given integer with pow(2,number_of_bits-1)
    // and print the result
    return ((1 << number_of_bits) - 1) ^ n;
}

int main()
{
    unsigned int n = 22;
    cout << onesComplement(n);
    return 0;
}
Java 
// Java program to find 1's complement of n.
class GFG {

    static int onesComplement(int n)
    {

        // Find number of bits in the
        // given integer
        int number_of_bits
            = (int)(Math.floor(Math.log(n) / Math.log(2)))
              + 1;

        // XOR the given integer with pow(2,
        // number_of_bits-1 and print the result
        return ((1 << number_of_bits) - 1) ^ n;
    }

    // Driver code
    public static void main(String[] args)
    {
        int n = 22;

        System.out.print(onesComplement(n));
    }
}

// This code is contributed by Anant Agarwal.
Python 
# Python3 program to find
# 1's complement of n.
import math


def onesComplement(n):

    # Find number of bits in
    # the given integer
    number_of_bits = (int)(math.floor(math.log(n) /
                                      math.log(2))) + 1

    # XOR the given integer with pow(2,
    # number_of_bits-1 and print the result
    return ((1 << number_of_bits) - 1) ^ n


# Driver code
n = 22
print(onesComplement(n))

# This code is contributed by Anant Agarwal.
C# 
// C# program to find 1's complement of n.
using System;

class GFG {

    static int onesComplement(int n)
    {

        // Find number of bits in the given integer
        int number_of_bits
            = (int)(Math.Floor(Math.Log(n) / Math.Log(2)))
              + 1;

        // XOR the given integer with pow(2,
        // number_of_bits-1 and print the result
        return ((1 << number_of_bits) - 1) ^ n;
    }

    // Driver code
    public static void Main()
    {

        int n = 22;

        Console.WriteLine(onesComplement(n));
    }
}

// This code is contributed by Anant Agarwal.
JavaScript 
<script>

// Javascript program to find
// 1's complement of n.

// Function to check whether a
// number is power of 2 or not
function onesComplement(n)
    {
          
        // Find number of bits in the 
        // given integer
        let number_of_bits = 
               (Math.floor(Math.log(n) /
                           Math.log(2))) + 1;
  
        // XOR the given integer with pow(2,
        // number_of_bits-1 and print the result
        return ((1 << number_of_bits) - 1) ^ n;
    }

// driver program 

    let n = 22;
          
    document.write(onesComplement(n));
            
</script>
PHP 
<?php
// PHP program to find 1's
// complement of n.

function Log2($x)
{ 
    return (log10($x) / log10(2));
}

function onesComplement($n)
{
    
// Find number of bits in 
// the given integer
$number_of_bits = floor(log2($n)) + 1;

// XOR the given integer with 
// number_of_bits-1 and print the result 
return ((1 << $number_of_bits) - 1) ^ $n;
}

// Driver Code
$n = 22;
echo onesComplement($n);

// This code is contributed by ihritik
?>

Output
9


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