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Print first n numbers with exactly two set bits

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Given a number n, print first n positive integers with exactly two set bits in their binary representation.
Examples :

Input: n = 3
Output: 3 5 6
The first 3 numbers with two set bits are 3 (0011),
5 (0101) and 6 (0110)
Input: n = 5
Output: 3 5 6 9 10 12

A Simple Solution is to consider all positive integers one by one starting from 1. For every number, check if it has exactly two sets bits. If a number has exactly two set bits, print it and increment count of such numbers.
An Efficient Solution is to directly generate such numbers. If we clearly observe the numbers, we can rewrite them as given below pow(2,1)+pow(2,0), pow(2,2)+pow(2,0), pow(2,2)+pow(2,1), pow(2,3)+pow(2,0), pow(2,3)+pow(2,1), pow(2,3)+pow(2,2), ………
All numbers can be generated in increasing order according to higher of two set bits. The idea is to fix higher of two bits one by one. For current higher set bit, consider all lower bits and print the formed numbers.

C++

// C++ program to print first n numbers
// with exactly two set bits
#include <iostream>
using namespace std;
 
// Prints first n numbers with two set bits
void printTwoSetBitNums(int n)
{
    // Initialize higher of two sets bits
    int x = 1;
 
    // Keep reducing n for every number
    // with two set bits.
    while (n > 0)
    {
        // Consider all lower set bits for
        // current higher set bit
        int y = 0;
        while (y < x)
        {
            // Print current number
            cout << (1 << x) + (1 << y) << " ";
 
            // If we have found n numbers
            n--;
            if (n == 0)
                return;
 
            // Consider next lower bit for current
            // higher bit.
            y++;
        }
 
        // Increment higher set bit
        x++;
    }
}
 
// Driver code
int main()
{
    printTwoSetBitNums(4);
 
    return 0;
}

                    

Java

// Java program to print first n numbers
// with exactly two set bits
import java.io.*;
 
class GFG
{
    // Function to print first n numbers with two set bits
    static void printTwoSetBitNums(int n)
    {
        // Initialize higher of two sets bits
        int x = 1;
  
        // Keep reducing n for every number
        // with two set bits
        while (n > 0)
        {
            // Consider all lower set bits for
            // current higher set bit
            int y = 0;
            while (y < x)
            {
                // Print current number
                System.out.print(((1 << x) + (1 << y)) +" ");
  
                // If we have found n numbers
                n--;
                if (n == 0)
                    return;
  
                // Consider next lower bit for current
                // higher bit.
                y++;
            }
  
            // Increment higher set bit
            x++;
        }
    }
     
    // Driver program
    public static void main (String[] args)
    {
        int n = 4;
        printTwoSetBitNums(n);
    }
}
 
// This code is contributed by Pramod Kumar

                    

Python3

# Python3 program to print first n
# numbers with exactly two set bits
 
# Prints first n numbers
# with two set bits
def printTwoSetBitNums(n) :
 
    # Initialize higher of
    # two sets bits
    x = 1
 
    # Keep reducing n for every
    # number with two set bits.
    while (n > 0) :
     
        # Consider all lower set bits
        # for current higher set bit
        y = 0
        while (y < x) :
         
            # Print current number
            print((1 << x) + (1 << y),
                          end = " " )
 
            # If we have found n numbers
            n -= 1
            if (n == 0) :
                return
 
            # Consider next lower bit
            # for current higher bit.
            y += 1
         
        # Increment higher set bit
        x += 1
     
# Driver code
printTwoSetBitNums(4)
 
# This code is contributed
# by Smitha

                    

C#

// C# program to print first n numbers
// with exactly two set bits
using System;
 
class GFG
    {
         
    // Function to print first n
    // numbers with two set bits
    static void printTwoSetBitNums(int n)
    {
         
        // Initialize higher of
        // two sets bits
        int x = 1;
   
        // Keep reducing n for every
        // number with two set bits
        while (n > 0)
        {
             
            // Consider all lower set bits
            // for current higher set bit
            int y = 0;
            while (y < x)
            {
                 
                // Print current number
                Console.Write(((1 << x) +
                                (1 << y)) +" ");
   
                // If we have found n numbers
                n--;
                if (n == 0)
                    return;
   
                // Consider next lower bit
                // for current higher bit.
                y++;
            }
   
            // Increment higher set bit
            x++;
        }
    }
      
    // Driver program
    public static void Main()
    {
        int n = 4;
        printTwoSetBitNums(n);
    }
}
  
// This code is contributed by Anant Agarwal.

                    

Javascript

<script>
 
// Javascript program to print first n numbers
// with exactly two set bits
 
// Prints first n numbers with two set bits
function printTwoSetBitNums(n)
{
 
    // Initialize higher of two sets bits
    let x = 1;
 
    // Keep reducing n for every number
    // with two set bits.
    while (n > 0)
    {
     
        // Consider all lower set bits for
        // current higher set bit
        let y = 0;
        while (y < x)
        {
         
            // Print current number
            document.write((1 << x) + (1 << y) + " ");
 
            // If we have found n numbers
            n--;
            if (n == 0)
                return;
 
            // Consider next lower bit for current
            // higher bit.
            y++;
        }
 
        // Increment higher set bit
        x++;
    }
}
 
// Driver code
printTwoSetBitNums(4);
 
// This code is contributed by Mayank Tyagi
</script>

                    

PHP

<?php
// PHP program to print
// first n numbers with
// exactly two set bits
 
// Prints first n numbers
// with two set bits
function printTwoSetBitNums($n)
{
    // Initialize higher of
    // two sets bits
    $x = 1;
 
    // Keep reducing n for
    // every number with
    // two set bits.
    while ($n > 0)
    {
        // Consider all lower set
        // bits for current higher
        // set bit
        $y = 0;
        while ($y < $x)
        {
            // Print current number
            echo (1 << $x) + (1 << $y), " ";
 
            // If we have found n numbers
            $n--;
            if ($n == 0)
                return;
 
            // Consider next lower
            // bit for current
            // higher bit.
            $y++;
        }
 
        // Increment higher set bit
        $x++;
    }
}
 
// Driver code
printTwoSetBitNums(4);
 
// This code is contributed by Ajit
?>

                    

Output : 
 

3 5 6 9


Time Complexity : O(n)

Auxiliary Space: O(1)

Approach#2: Using while and join


The approach is to start from the integer 3 and check whether the number of set bits in its binary representation is equal to 2 or not. If it has exactly 2 set bits, then add it to the list of numbers with 2 set bits until the list has n elements.

Algorithm

1. Initialize an empty list res to store the integers with exactly two set bits.
2. Initialize an integer variable i to 3.
3. While the length of the list res is less than n, do the following:
a. Check whether the number of set bits in the binary representation of i is equal to 2 or not using the count() method of the string.
b. If the number of set bits is equal to 2, then append i to the list res.
c. Increment i by 1.
4. Return the list res.

C++

#include <iostream>
#include <vector>
using namespace std;
 
int countSetBits(int num) {
    int count = 0;
    while (num > 0) {
        count += num & 1;
        num >>= 1;
    }
    return count;
}
 
vector<int> numbersWithTwoSetBits(int n) {
    vector<int> res;
    int i = 3;
    while (res.size() < n) {
        if (countSetBits(i) == 2) {
            res.push_back(i);
        }
        i++;
    }
    return res;
}
 
int main() {
    int n = 3;
    vector<int> result = numbersWithTwoSetBits(n);
 
    cout << "Result: ";
    for (int i = 0; i < result.size(); i++) {
        cout << result[i] << " ";
    }
    cout << endl;
 
    return 0;
}

                    

Java

// Java program for the above approach
import java.util.ArrayList;
import java.util.List;
 
public class GFG {
    // Function to count the number of set bits (binary 1s)
    // in an integer
    static int countSetBits(int num)
    {
        int count = 0;
        while (num > 0) {
            count += num & 1; // Increment count if the last
                              // bit is set (1)
            num >>= 1; // Right shift to check the next bit
        }
        return count;
    }
 
    // Function to generate 'n' numbers with exactly two set
    // bits in their binary representation
    static List<Integer> numbersWithTwoSetBits(int n)
    {
        List<Integer> res = new ArrayList<>();
        int i = 3; // Start from 3 as the first number with
                   // two set bits
        while (res.size() < n) {
            if (countSetBits(i)
                == 2) { // Check if the number has exactly
                        // two set bits
                res.add(
                    i); // Add the number to the result list
            }
            i++; // Move to the next number
        }
        return res;
    }
 
    public static void main(String[] args)
    {
        int n = 3; // Number of numbers with two set bits to
                   // generate
        List<Integer> result = numbersWithTwoSetBits(
            n); // Get the generated numbers
 
        for (int num : result) {
            System.out.print(
                num + " "); // Display the generated numbers
        }
        System.out.println();
    }
}
 
// This code is contributed by Susobhan Akhuli

                    

Python3

def numbersWithTwoSetBits(n):
    res = []
    i = 3
    while len(res) < n:
        if bin(i).count('1') == 2:
            res.append(i)
        i += 1
    return res
n = 3
result = numbersWithTwoSetBits(n)
output_string = ' '.join(str(x) for x in result)
print(output_string)

                    

C#

using System;
using System.Collections.Generic;
 
class Program
{
    // Function to count the number of set bits (binary 1s) in an integer
    static int CountSetBits(int num)
    {
        int count = 0;
        while (num > 0)
        {
            count += num & 1; // Increment count if the last bit is set (1)
            num >>= 1; // Right shift to check the next bit
        }
        return count;
    }
 
    // Function to generate 'n' numbers with exactly two set bits in their binary representation
    static List<int> NumbersWithTwoSetBits(int n)
    {
        List<int> res = new List<int>();
        int i = 3; // Start from 3 as the first number with two set bits
        while (res.Count < n)
        {
            if (CountSetBits(i) == 2) // Check if the number has exactly two set bits
            {
                res.Add(i); // Add the number to the result list
            }
            i++; // Move to the next number
        }
        return res;
    }
 
    static void Main(string[] args)
    {
        int n = 3; // Number of numbers with two set bits to generate
        List<int> result = NumbersWithTwoSetBits(n); // Get the generated numbers
 
        Console.Write("Result: ");
        foreach (int num in result)
        {
            Console.Write(num + " "); // Display the generated numbers
        }
        Console.WriteLine();
    }
}

                    

Javascript

// Javascript program for the above approach
 
// Function to count the number of set bits (binary 1s)
// in an integer
function countSetBits(num) {
    let count = 0;
    while (num > 0) {
        count += num & 1; // Increment count if the last
                          // bit is set (1)
        num >>= 1; // Right shift to check the next bit
    }
    return count;
}
 
// Function to generate 'n' numbers with exactly two set
// bits in their binary representation
function numbersWithTwoSetBits(n) {
    let res = [];
    let i = 3; // Start from 3 as the first number with
               // two set bits
    while (res.length < n) {
        if (countSetBits(i) === 2) { // Check if the number has exactly
                                     // two set bits
            res.push(i); // Add the number to the result list
        }
        i++; // Move to the next number
    }
    return res;
}
 
// Number of numbers with two set bits to generate
let n = 3;
// Get the generated numbers
let result = numbersWithTwoSetBits(n);
 
// Display the generated numbers
console.log(result.join(' '));
 
// This code is contributed by Susobhan Akhuli

                    

Output
3 5 6







Time Complexity: O(n log n), where n is the number of integers with exactly two set bits. This is because we are checking the number of set bits in the binary representation of each integer, which takes O(log n) time.

Space Complexity: O(n), where n is the number of integers with exactly two set bits. This is because we are storing the list of integers with two set bits in memory.
 



Last Updated : 12 Dec, 2023
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