https://leetcode.com/problems/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.

一个正整数n与n-1总是相差一个不同位置的bit位，所以 n&^(n-1)一次就会消去一个‘1’bit，这样循环下去，有多少个’1’bit只需要执行这么多次与操作可以了，这比暴力统计每个bit位要快。

int hammingWeight(uint32_t n) {int cnt = 0;while(n > 0) {n = n & (n-1);++cnt;}return cnt;
}