Leetcode 0191. 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.

Programming Skills I Easy

Description

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.

Example 1:

Input: n = 00000000000000000000000000001011
Output: 3

• The input binary string 00000000000000000000000000001011 has a total of three ‘1’ bits.

Example 2:

Input: n = 00000000000000000000000010000000
Output: 1

• The input binary string 00000000000000000000000010000000 has a total of one ‘1’ bit.

Example 3:

Input: n = 11111111111111111111111111111101
Output: 31

• 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?

Solution

class Solution:
    # Iteration
    # Time Complexity: BigO(N)
    # Space Complexity: BigO(1)
    def hammingWeight(self, n: int) -> int:
        count = 0
        while n:
            count += n & 1
            n >>= 1
        return count

• Bitwise AND, 1011 & 0001: return True when both bit has 1

  1011 & 0001 = 0001
  101 & 001 = 001
  10 & 01 = 00
  1 & 1 = 1
  

• bin(n).count("1") and f"{n:b}".count("1") can be used
• bit_count()

/**
 * Iteration
 * Time Complexity: BigO(N)
 * Space Complexity: BigO(1)
 */
function hammingWeight(n: number): number {
  return n
    .toString(2)
    .split("")
    .reduce((acc, cur) => {
      if (cur === "1") return (acc += 1);
      else return (acc += 0);
    }, 0);
}