Codeforces 1185G2 Playlist for Polycarp (hard version) 背包，暴力

题意及思路：https://www.cnblogs.com/Als123/p/11061147.html

代码：

#include <bits/stdc++.h>
#define LL long long
#define INF 0x3f3f3f3f
#define db double
#define pii pair<int, int>
using namespace std;
const LL mod = 1e9 + 7;
LL mul(LL x, LL y) {
	return ((LL)x * y) % mod;
}
LL add(LL x, LL y) {
	return ((LL)x + y) % mod;
}
LL dp[55][55][55][3], sum[55][55][55], ssum[3];
LL f[3][55][2505], jc[55], f1[55][55][2505];
vector<int> a[3];
int n, m;
LL ans = 0;
void init(int x) {
	jc[0] = 1;
	for (int i = 1; i <= x; i++) {
		jc[i] = mul(jc[i - 1], i);
	}
	dp[1][0][0][0] = dp[0][1][0][1] = dp[0][0][1][2] = 1;
	for (int i = 0; i <= x; i++)
		for (int j = 0; j <= x; j++)
			for (int k = 0; k <= x; k++) {
				for (int t = 0; t < 3; t++) {
					if(t != 0) dp[i + 1][j][k][0] += dp[i][j][k][t];
					if(t != 1) dp[i][j + 1][k][1] += dp[i][j][k][t];
					if(t != 2) dp[i][j][k + 1][2] += dp[i][j][k][t];
				}
				sum[i][j][k] = mul(mul(jc[i], mul(jc[j], jc[k])), add(add(dp[i][j][k][0], dp[i][j][k][1]), dp[i][j][k][2]));
			}
}
void init1() {
	for (int flag = 0; flag < 3; flag++) {
		f[flag][0][0] = 1;
		for (int i = 0; i < a[flag].size(); i++)
			for (int j = i + 1; j >= 1; j--)
				for (int k = ssum[flag]; k >= a[flag][i]; k--)
					f[flag][j][k] = add(f[flag][j][k], f[flag][j - 1][k - a[flag][i]]);
	}
	for (int i = 0; i <= a[0].size(); i++)
		for (int j = 0; j <= a[1].size(); j++)
			for (int k = 0; k <= ssum[0]; k++) {
				for (int t = 0; t <= ssum[1]; t++) {
					f1[i][j][k + t] = add(f1[i][j][k + t], mul(f[0][i][k], f[1][j][t]));
				}
			}
}
void solve() {
	for (int i = 0; i <= a[2].size(); i++)
		for (int j = 0; j <= a[0].size(); j++)
			for (int k = 0; k <= a[1].size(); k++) {
				for (int t = 0; t <= ssum[2]; t++) {
					int tmp1 = m - t;
					ans = add(ans, mul(sum[j][k][i], mul(f1[j][k][tmp1], f[2][i][t])));
				}
			}
	printf("%lld\n", ans);
}
int main() {
	int t, x;
	init(50);
	scanf("%d%d", &n, &m);
	for (int i = 1; i <= n; i++) {
		scanf("%d%d", &t, &x);
		a[x - 1].push_back(t);
		ssum[x - 1] += t;
	}
	if(a[0].size() > a[2].size()) {
		swap(a[0], a[2]);
		swap(ssum[0], ssum[2]);
	}
	if(a[1].size() > a[2].size()) {
		swap(a[1], a[2]);
		swap(ssum[1], ssum[2]);
	}
	init1();
	solve();
}