P5023 [NOIP2018 提高组] 填数游戏

P5023 NOIP2018 提高组 填数游戏

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#include <stdio.h>
typedef long long LL;
const int mod = 1e9 + 7;
const int a_nn[] = {0, 2, 12, 112, 912, 7136, 56768, 453504, 3626752};
const int a_nm[] = {0, 4, 36, 336, 2688, 21312, 170112, 1360128, 10879488};
int qpow(int b, int p) {
	int res = 1;
	while(p) {
		if(p & 1) res = (LL)res * b % mod;
		b = (LL)b * b % mod, p >>= 1;
	}
	return res;
}
int main() {
	int n, m; scanf("%d%d", &n, &m);
	if(n > m) n ^= m ^= n ^= m;
	if(n == 1) printf("%d\n", qpow(2, m));
	else if(n == m) printf("%d\n", a_nn[n]);
	else if(n + 1 == m) printf("%d\n", a_nm[n]);
	else printf("%d\n", int((LL)a_nm[n] * qpow(3, m - n - 1) % mod));
	return 0;
}