191. Number of 1 Bits

Write a function that takes an unsigned integer and returns the number of ‘1’ bits it has (also known as the Hamming weight).

Note:

• Note that in some languages, such as Java, there is no unsigned integer type. In this case, the input will be given as a signed integer type. It should not affect your implementation, as the integer’s internal binary representation is the same, whether it is signed or unsigned.
• In Java, the compiler represents the signed integers using 2’s complement notation. Therefore, in Example 3, the input represents the signed integer. -3.

Follow up: If this function is called many times, how would you optimize it?

• cache can be solution but not perfect
• input range is [0 … 2^32-1] so it’s too much data for caching
  • we need 2^32 - 1 * uint32_t memory in case
• we can use cache with lru or something else..
  • but worst!!
  • in this case average cache hit rate wold be memory size / 2^32 - 1
  • and also we know we have to use hash funtions
    • when we use hash funtions, we have to make hash key, so have to iterate all bits O(n)
  • in below solution, we iterate n but space O(1)

class Solution {
public:
    int hammingWeight(uint32_t n) {
        int res = 0;
        while(n) {
            if(n & 1) res++;
            n>>=1;
        }
        return res;
    }
};