bzoj1485: [HNOI2009]有趣的数列

#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<iostream>

using namespace std;

void setIO(const string& s) {
freopen((s + ".in").c_str(), "r", stdin);
freopen((s + ".out").c_str(), "w", stdout);
}
template<typename Q> Q read(Q& x) {
static char c, f;
for(f = 0; c = getchar(), !isdigit(c); ) if(c == '-') f = 1;
for(x = 0; isdigit(c); c = getchar()) x = x * 10 + c - '0';
if(f) x = -x;
return x;
}
template<typename Q> Q read() {
static Q x; read(x); return x;
}

typedef long long LL;
const int LIM = 2000000, N = 1000000;

int primes
, tot, mn[LIM+1];
bool flag[LIM+1];

void get_prime(int n) {
for(int i = 2; i <= n; i++) {
if(!flag[i]) primes[tot] = i, mn[i] = tot++;
for(int j = 0; j < tot; j++) {
int k = primes[j];
if((LL) i * k > n) break;
flag[i * k] = 1;
mn[i * k] = j;
if(i % k == 0) break;
}
}
}

int num
;

void add(int x, int sign) {
while(x != 1) {
num[mn[x]] += sign;
x /= primes[mn[x]];
}
}

LL qpow(LL a, int b, int p) {
a %= p;
for(LL c = 1; ; (a *= a) %= p) {
if(b & 1) (c *= a) %= p;
if(!(b >>= 1)) return c;
}
}

int main() {
#ifdef DEBUG
freopen("in.txt", "r", stdin);
freopen("out.txt", "w", stdout);
#endif

int n, p;
scanf("%d%d", &n, &p);
get_prime(n * 2);
for(int i = n * 2; i > n; i--) add(i, 1);
for(int i = n; i > 1; i--) add(i, -1);
add(n + 1, -1);

int res = 1;
for(int i = 0; i < tot; i++) if(num[i]) {
res = (LL) res * qpow(primes[i], num[i], p) % p;
}
printf("%d\n", res);

return 0;
}