Ultraviolet A 11426-GCD-extreme (II) (number theory)

Link to the question: Ultraviolet A 11426-GCD-extreme

Question: Give N, ask Σ I! = Jngcd (I, j)

Solution: F (n) = gcd (1, N) + gcd (2, n) +? + Gcd (n? 1, n)
S (n) = F (2) + f (3) +? + F (N)
S (n) = S (n? 1) + f (N)
The problem is converted to how to calculate F (N). For a n, the enumerated factor can multiply the number of upper.

#include <cstdio>#include <cstring>const int N = 4000000;typedef long long ll;int phi[N+5];ll s[N+5], f[N+5];void phi_table(int n) {    for (int i = 2; i <= n; i++)        phi[i] = 0;    phi[1] = 1;    for (int i = 2; i <= n; i++) {        if (!phi[i]) {            for (int j = i; j <= n; j += i) {                if (!phi[j])                    phi[j] = j;                phi[j] = phi[j] / i * (i-1);            }        }    }}int main () {    phi_table(N);    memset(f, 0, sizeof(f));    for (int i = 1; i <= N; i++) {        for (int j = i * 2; j <= N; j += i) {            f[j] += i * phi[j/i];        }    }    s[2] = f[2];    for (int i = 3; i <= N; i++)        s[i] = s[i-1] + f[i];    int n;    while (scanf("%d", &n) == 1 && n) {        printf("%lld\n", s[n]);    }    return 0;}