191. Number of 1 Bits 二进制1的个数

题目链接
tag:

• easy；
• Bit；

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

Example 1:

Input: 00000000000000000000000000001011
Output: 3
Explanation: The input binary string **00000000000000000000000000001011** has a total of three '1' bits.

Example 2:

Input: 00000000000000000000000010000000
Output: 1
Explanation: The input binary string 00000000000000000000000010000000 has a total of one '1' bit.

Example 3:

Input: 11111111111111111111111111111101
Output: 31
Explanation: The input binary string 11111111111111111111111111111101 has a total of thirty one '1' bits.

Note:

• Note that in some languages such as Java, there is no unsigned integer type. In this case, the input will be given as signed integer type and should not affect your implementation, as the internal binary representation of the integer 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?

思路：
  把一个整数减去1，然后与原整数做与运算，会把该整数最右边的一个1变成0，要计算1的个数，就循环上述操作，直到这个数为0为止。代码如下：

class Solution {
public:
    int hammingWeight(uint32_t n) {
        int res = 0;
        while (n) {
            n = n & (n - 1);
            ++res;
        }
        return res;
    }
};