Created
September 9, 2015 22:48
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#include <iostream> | |
#include <math.h> | |
using namespace std; | |
int StartingAt(int n); | |
int main() | |
{ | |
cout << StartingAt(1); | |
return 0; | |
} | |
int mul_mod(long a, long b, int m) | |
{ | |
return (int)((a * b) % m); | |
} | |
// return the inverse of x mod y | |
int inv_mod(int x, int y) | |
{ | |
int q = 0; | |
int u = x; | |
int v = y; | |
int a = 0; | |
int c = 1; | |
int t = 0; | |
do | |
{ | |
q = v / u; | |
t = c; | |
c = a - q * c; | |
a = t; | |
t = u; | |
u = v - q * u; | |
v = t; | |
} | |
while (u != 0); | |
a = a % y; | |
if (a < 0) a = y + a; | |
return a; | |
} | |
// return (a^b) mod m | |
int pow_mod(int a, int b, int m) | |
{ | |
int r = 1; | |
int aa = a; | |
while (true) | |
{ | |
if ((b & 0x01) != 0) r = mul_mod(r, aa, m); | |
b = b >> 1; | |
if (b == 0) break; | |
aa = mul_mod(aa, aa, m); | |
} | |
return r; | |
} | |
// return true if n is prime | |
bool is_prime(int n) | |
{ | |
if ((n % 2) == 0) return false; | |
int r = (int)(sqrt((double)n)); | |
for (int i = 3; i <= r; i += 2) | |
{ | |
if ((n % i) == 0) return false; | |
} | |
return true; | |
} | |
// return the prime number immediately after n | |
int next_prime(int n) | |
{ | |
do | |
{ | |
n++; | |
} | |
while (!is_prime(n)); | |
return n; | |
} | |
int StartingAt(int n) | |
{ | |
int av = 0; | |
int vmax = 0; | |
int N = (int)((n + 20) * log(10.) / log(2.)); | |
int num = 0; | |
int den = 0; | |
int kq = 0; | |
int kq2 = 0; | |
int t = 0; | |
int v = 0; | |
int s = 0; | |
double sum = 0.0; | |
for (int a = 3; a <= (2 * N); a = next_prime(a)) | |
{ | |
vmax = (int)(log(2. * N) / log((double)a)); | |
av = 1; | |
for (int i = 0; i < vmax; ++i) av = av * a; | |
s = 0; | |
num = 1; | |
den = 1; | |
v = 0; | |
kq = 1; | |
kq2 = 1; | |
for (int k = 1; k <= N; ++k) | |
{ | |
t = k; | |
if (kq >= a) | |
{ | |
do | |
{ | |
t = t / a; | |
--v; | |
} | |
while ((t % a) == 0); | |
kq = 0; | |
} | |
++kq; | |
num = mul_mod(num, t, av); | |
t = (2 * k - 1); | |
if (kq2 >= a) | |
{ | |
if (kq2 == a) | |
{ | |
do | |
{ | |
t = t / a; | |
++v; | |
} | |
while ((t % a) == 0); | |
} | |
kq2 -= a; | |
} | |
den = mul_mod(den, t, av); | |
kq2 += 2; | |
if (v > 0) | |
{ | |
t = inv_mod(den, av); | |
t = mul_mod(t, num, av); | |
t = mul_mod(t, k, av); | |
for (int i = v; i < vmax; ++i) t = mul_mod(t, a, av); | |
s += t; | |
if (s >= av) s -= av; | |
} | |
} | |
t = pow_mod(10, n - 1, av); | |
s = mul_mod(s, t, av); | |
sum = (sum + (double)s / (double)av); | |
sum -= floor(sum); | |
} | |
return (int)(sum * 1e9); | |
} |
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