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).

For example, the 32-bit integer ’11' has binary representation

00000000000000000000000000001011

,
so the function should return 3.

思路有两种：

第一种：

将n二进制化，在二进制化的过程中统计1的个数

java中没有无符号整数的表示，所以不是很容易，int 的范围是-2147483648 ~ 2147483647

第二种：

利用按位与，统计1的个数

public class Solution {
// you need to treat n as an unsigned value
public int hammingWeight(int n) {
int count = 0;
int tmp = 1;
int i = 0 ;
while(i < 32){
if((n & tmp) != 0 ){
count ++;
}
tmp = tmp << 1;
i++;
}
return count;
}
}

注意：& 的优先级低于 != ，所以n&tmp要加括号
Runtime: 220
ms