## Number of 1 Bits

### 描述

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.

### 分析

1. 输入值n可能为负数，但应视其为无符号整数，因为Java中实际上是没有无符号整数的
2. 使用无符号右移操作符>>>，可以忽略符号位

### 解法1

// Number of 1 Bits
// Time Complexity: O(32), Space Complexity: O(1)
public class Solution {
// you need to treat n as an unsigned value
public int hammingWeight(int n) {
int count = 0;
for (int i = 0; i < 32; ++i) {
count += n & 1;
n >>>= 1;
}
return count;
}
}

### 解法2

// Number of 1 Bits
// Time Complexity: O(number of 1), Space Complexity: O(1)
public class Solution {
// you need to treat n as an unsigned value
public int hammingWeight(int n) {
int count = 0;
while (n != 0) {
++count;
n &= n - 1;
}
return count;
}
}