191. Number of 1 Bits

Description

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

For example, the 32-bit integer ’11' has binary representation 00000000000000000000000000001011, so the function should return 3.

Solution

注意unsigned移位。

Iteration, O(1), S(1)

public class Solution {
    // you need to treat n as an unsigned value
    public int hammingWeight(int n) {
        int digits = 0;
        
        for (int i = 0; i < 32; ++i) {
            digits += n & 1;
            n >>>= 1;
        }
        
        return digits;
    }
}

Bit-manipulation trick, O(1), S(1)

public class Solution {
    // you need to treat n as an unsigned value
    public int hammingWeight(int n) {
        int digits = 0;
        
        while (n != 0) {
            ++digits;
            n &= n - 1;
        }
        
        return digits;
    }
}