Power of 2, solving a google interview question. Playing with bits.

Question : Given an integer, write a function to determine if it is a power of two. (Range: 1 - 2^31-1)

Eg: Input : 16, Output : true since 2^4 = 16.
Input : 18, Output : false.

Brute Force

So the obvious brute force approach will be to just divide 2 and check if it reaches 1.

var powerOftwo = function(n){ if(n<=0) return false; while(n%2 == 0) n = n/2; return n == 1; } 

Percompute and Use Set

Since we're give that the range is in between 0 - 2^31-1, we could use this to precompute various possible output, save them into a set and cross check:

 let set = new Set(); for(let i=0;i<31;i++){ set.add(Math.pow(2,i)); } console.log(set); var powerOfTwo = function(n){ if(set.has(n)) return true; return false; } 

Now that we know what we want to accomplish, let's think about what google might expect from us.

Tip : One common pattern I noticed is that whenever a company asks a question which looks easy on understand and easy to implement, that question has to be somehow related to bit manipulation.

So how do we relate power of 2 and bits

As we know, for a computer everything boils down to a combination of 0's and 1's including numbers. So let's see how the numbers which represent 2^n for n=0 to 31 are represented in bit form.

So the common pattern here is that the numbers that are power of 2's have only 1 bit set to 1 and rest of it is 0.

So how do we use this to our advantage ? Let's look at what operations we have to work with.

 &: and operator 1&1 = 1, 0 & anything = 0 |: or operator 0&0 = 0, 1 & anything = 1 ^: xor operator x^x = 0, x^n = 1 ~: negate ~1 = 0 | ~0 = 1 

Tip: if the question is regarding bit manipulation, first try solving with & and ^ operators

Let's try solving this with & operator. But what should we & it with ?

Let's generate a number, if a number is odd, +1/-1 will generate an even number and vice versa.

Now that we have "&" operator and 2 numbers let put them together.

 even number : 6 = 110 +1 : 7 = 111 -1 : 5 = 101 6&7 = 6 6&5 = 4 multiple of 2 : 16 = 10000 +1 : 17 = 10001 -1 : 15 = 01111 16&17 = 16 16&15 = 0 

Here comes our solution, 16&15 becomes 0 since as proved earlier that a number if it's power of 2, has only 1 bit set, so n-1 will set all previous bit's from current position to 1.

eg:

 8 : 1000 7: 0111 8&7 = 0 16: 10000 15: 01111 16&15 = 0 32: 100000 31: 011111 32&31 = 0 

But at the same time if we use an odd number

eg:

 17: 10001 16: 10000 17&16 = 16 

From this we can say that n&(n-1) = 0, if n is power of 2.

Due to this our code boiled down to :

 var powerOfTwo = function(n){ n = n&(n-1); return n == 0; } 

Amazing isn't it ? How few bit manipulation cause such drastic results aand lead us to a more concise readable code.

If you want to practice something similar, solve this :

Question : Count the number of ones in the binary representation of the given number.

Comment your solution for this question :)

I hope you liked my explanation comment down below if I messed up somewhere.

github : https://github.com/AKHILP96/Data-Structures-and-Algorithms/blob/master/problems/powerOfTwo.js