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Check whether the number has only first and last bits set
  • Difficulty Level : Easy
  • Last Updated : 22 Mar, 2021

Given a positive integer n. The problem is to check whether only the first and last bits are set in the binary representation of n.
Examples: 
 

Input : 9
Output : Yes
(9)10 = (1001)2, only the first and
last bits are set.

Input : 15
Output : No
(15)10 = (1111)2, except first and last
there are other bits also which are set.

 

Approach: Following are the steps:
 

  1. If n == 1, return “Yes”.
  2. Else check whether n-1 is a power of 2. Refer this post.

 

C++




// C++ to check whether the number has only
// first and last bits set
#include <bits/stdc++.h>
 
using namespace std;
 
// function to check whether 'n'
// is a power of 2 or not
bool powerOfTwo(unsigned int n)
{
    return (!(n & n-1));
}
 
// function to check whether the number has only
// first and last bits set
bool onlyFirstAndLastAreSet(unsigned int n)
{
    if (n == 1)
        return true;
    if (n == 2)
        return false;
    return powerOfTwo(n-1);   
}
 
// Driver program to test above
int main()
{
    unsigned int n = 9;
     
    if (onlyFirstAndLastAreSet(n))
        cout << "Yes";
    else
        cout << "No";   
     
    return 0;
}

Java




// Java program to check whether the
// number has only first and last
// bits set
import java.util.*;
 
class GFG
{
    // function to check whether 'n'
    // is a power of 2 or not
    static boolean powerOfTwo(int n)
    {
        return ((n & n - 1) == 0);
    }
 
    // function to check whether the number has 
    // only first and last bits set
    static boolean onlyFirstAndLastAreSet(int n)
    {
        if (n == 1)
            return true;
     
        return powerOfTwo(n-1);
    }
 
    // Driver program to test above
    public static void main (String[] args) {
        
       int n = Integer.parseUnsignedInt("9");
     
        if (onlyFirstAndLastAreSet(n))
            System.out.println("Yes");
        else
            System.out.println("No");
    }
}
 
/* This code is contributed by Mr. Somesh Awasthi */

Python3




# Python3 program to check whether number
# has only first and last bits set
 
# function to check whether 'n'
# is a power of 2 or not
def powerOfTwo (n):
    return (not(n & n-1))
 
 
# function to check whether number 
# has only first and last bits set
def onlyFirstAndLastAreSet (n):
 
    if (n == 1):
        return True
    return powerOfTwo (n-1)
 
# Driver program to test above
n = 9
if (onlyFirstAndLastAreSet (n)):
    print('Yes')
else:
    print('No')
 
# This code is contributed by Shariq Raza

C#




// C# program to check whether the
// number has only first and last
// bits set
using System;
 
class GFG {
 
    // function to check whether 'n'
    // is a power of 2 or not
    static bool powerOfTwo(uint n)
    {
        return ((n & n - 1) == 0);
    }
 
    // function to check whether the number has
    // only first and last bits set
    static bool onlyFirstAndLastAreSet(uint n)
    {
        if (n == 1)
            return true;
 
        return powerOfTwo(n - 1);
    }
 
    // Driver program to test above
    public static void Main()
    {
 
        uint n = (uint)9;
 
        if (onlyFirstAndLastAreSet(n))
            Console.WriteLine("Yes");
        else
            Console.WriteLine("No");
    }
 
}
 
// This code is contributed by Sam007

PHP




<?php
// PHP to check whether the number
// has only first and last bits set
 
// function to check whether 'n'
// is a power of 2 or not
function powerOfTwo($n)
{
    return (!($n & $n - 1));
}
 
// function to check whether
// the number has only first
// and last bits set
function onlyFirstAndLastAreSet($n)
{
    if ($n == 1)
        return true;
    if ($n == 2)
        return false;
    return powerOfTwo($n - 1);    
}
 
// Driver Code
$n = 9;
 
if (onlyFirstAndLastAreSet($n))
    echo "Yes";
else
    echo "No";
 
// This code is contributed
// by Sach_Code
?>

Javascript




<script>
 
// Javascript to check whether the number has only
// first and last bits set
 
// function to check whether 'n'
// is a power of 2 or not
function powerOfTwo(n)
{
    return (!(n & n-1));
}
 
// function to check whether the number has only
// first and last bits set
function onlyFirstAndLastAreSet(n)
{
    if (n == 1)
        return true;
    if (n == 2)
        return false;
    return powerOfTwo(n-1);    
}
 
// Driver program to test above
var n = 9;
 
if (onlyFirstAndLastAreSet(n))
    document.write("Yes");
else
    document.write("No");    
     
</script>

Output: 

Yes

This article is contributed by Ayush Jauhari. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
 




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