Given an array of 32-bit integers, sort the array in ascending order of the number of set bits (the number of 1s in the binary representation). Note the solution must sort the array in place.

If two numbers have the same number of set bits, use the integer value to determine the order.

e.g. Given [7, 6, 15, 8]

• 7 has 3 set bits (000…0111)
• 6 has 2 set bits (000…0110)
• 15 has 4 set bits (000…1111)
• 8 has 1 set bit (000…1000)

Test data: [3, 8, 3, 6, 5, 7, 9, 1] => [1, 8, 3, 3, 5, 6, 9, 7]

1. Simple Method: Loop through all bits in an integer, check if a bit is set and if it is, then increment the set bit count.

2. Brian Kernighan’s Algorithm: Subtracting 1 from a decimal number flips all the bits after the rightmost set bit(which is 1) including the rightmost set bit.

For example :

10 in binary is 00001010 
9 in binary is 00001001 
8 in binary is 00001000 
7 in binary is 00000111 

So if we subtract a number by 1 and do it bitwise & with itself (n & (n-1)), we unset the rightmost set bit. If we do n & (n-1) in a loop and count the number of times the loop executes, we get the set bit count. The beauty of this solution is the number of times it loops is equal to the number of set bits in a given integer.

Initialize count: = 0
   If integer n is not zero
      (a) Do bitwise & with (n-1)
 	and assign the value 
	back to n
          n: = n&(n-1)
      (b) Increment count by 1
      (c) go to step 2
   Else return count

See solutions in different languages in the "Templates" directory. Once you decide which language you'd like to use, simply open that directory in your favorite IDE, and you should be able to run the included unit tests "out of the box".

The recommended IDEs are as follows, but feel free to use whatever IDE you are comfortable with.

• C# - Microsoft Visual Studio
• Java - IntelliJ Idea (Community Edition is fine)
• JavaScript - Microsoft Visual Studio Code
• Kotlin - IntelliJ Idea (Community Edition is fine)
• TypeScript - Microsoft Visual Studio Code