复杂度\(O(n^\frac23)\)，计算\(1\simn\)的素数个数#definediv(a,b)(1.0*(a)/(b))#definehalf(x)(((x)-1)/2)i64Meissel_Lehmer(i64n){if(n<=3){returnmax(n-1,0LL);}longlongv=sqrtl(n);ints=(v+1)/2