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 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 above, the input represents the signed integer. -3.

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


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.



  • The input must be a binary string of length 32

191. Number of 1 Bits
struct Solution;

impl Solution {
    fn hammingWeight(n: u32) -> i32 {
        n.count_ones() as i32

fn test() {
    let n = 0b00000000000000000000000000001011;
    let res = 3;
    assert_eq!(Solution::hammingWeight(n), res);
    let n = 0b00000000000000000000000010000000;
    let res = 1;
    assert_eq!(Solution::hammingWeight(n), res);
    let n = 0b11111111111111111111111111111101;
    let res = 31;
    assert_eq!(Solution::hammingWeight(n), res);