公式：lowbit(x) = x & (-x)
前置知识：
-x = ~x + 1（补码 = 反码 + 1）
ps: u 和 v 互为反码，为任意 0 或 -1
bash
x uuuuuuuuuuuuuuuu10000
~x vvvvvvvvvvvvvvvv01111
~x+1 vvvvvvvvvvvvvvvv10000
-x vvvvvvvvvvvvvvvv10000
x&(-x) 000000000000000010000