Count number of 1's in binary representation

Question

Efficient way to count number of 1s in the binary representation of a number in O(1) if you have enough memory to play with. This is an interview question I found on an onli

Answer 1

Below are two simple examples (in C++) among many by which you can do this.

1. We can simply count set bits (1's) using __builtin_popcount().

   int numOfOnes(int x) { return __builtin_popcount(x); }

2. Loop through all bits in an integer, check if a bit is set and if it is then increment the count variable.

   int hammingDistance(int x) { int count = 0 for(int i = 0; i < 32; i++) if(x & (1 << i)) count++; return count; }

Hope this helps!