\((c\&(-c))\) --- 抹去更高位的 \(1\)
\((c/(c\&(-c)))\) --- 删除 \(c\)的低位零
\((1<<(c-1))\) --- \(2^{c-1}\)
\((w\&(1<<(c-1)))\) --- 取出 \(w\) 中 \(2^{c-1}\)
\(limit!=0\) --- 满足条件为1，否则为0
\(k|c\) --- 不进位相加
