content: check if a number is a power of 2 (#196)

* Check if a number is a power of 2

This representation may be easier to understand for some people.
If a number is power of two then `AND` operation between the number and its complement will result into the same number.

* Update binary.md

Co-authored-by: Yangshun Tay <tay.yang.shun@gmail.com>

contents/algorithms/binary.md

@@ -17,7 +17,7 @@ Some helpful utility snippets:
 - Set k<sup>th</sup> bit: `num |= (1 << k)`.
 - Turn off k<sup>th</sup> bit: `num &= ~(1 << k)`.
 - Toggle the k<sup>th</sup> bit: `num ^= (1 << k)`.
-- To check if a number is a power of 2, `num & num - 1 == 0`.
+- To check if a number is a power of 2, `(num & num - 1) == 0` or `(num & (-num)) == num`.
 
 ## Corner cases