191 Number of 1 Bits - Easy

Problem:

Write a function that takes the binary representation of 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.

Example 1:

Input: n = 00000000000000000000000000001011 Output: 3 Explanation: The input binary string 00000000000000000000000000001011 has a total of three '1' bits.

Example 2:

Input: n = 00000000000000000000000010000000 Output: 1 Explanation: The input binary string 00000000000000000000000010000000 has a total of one '1' bit.

Example 3:

Input: n = 11111111111111111111111111111101 Output: 31 Explanation: The input binary string 11111111111111111111111111111101 has a total of thirty one '1' bits.

Constraints:

• The input must be a binary string of length 32.

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

Problem Analysis:

• use bit manipulation
• O(1) time and space complexity. O(32) time complexity to be exact

Solutions:

class Solution:
    def hammingWeight(self, n: int) -> int:
        res = 0
        while n:
            res += n%2
            n = n >>1
        return res

Slightly faster below

class Solution:
    def hammingWeight(self, n: int) -> int:
        res = 0
        while n:
            n &= (n-1)
            res +=1
        return res

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