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55 lines
1.0 KiB
C++
55 lines
1.0 KiB
C++
#include <iostream>
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using std::cin;
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using std::cout;
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const char endl = '\n';
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const int N = 2005;
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const int mod = 1e9 + 7;
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int n, m, a[N], c[N][N], f[N], ans;
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int main() {
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std::ios::sync_with_stdio(false);
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cin.tie(nullptr);
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c[0][0] = 1;
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for (int i = 1; i < N; ++i) {
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c[i][0] = 1;
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for (int j = 1; j <= i; ++j) {
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c[i][j] = (static_cast<long long>(c[i - 1][j - 1]) + c[i - 1][j]) % mod;
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}
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}
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cin >> n >> m;
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for (int i = 1; i <= m; i++) {
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cin >> a[i];
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}
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for (int i = 0; i < n; i++) {
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f[i] = 1;
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for (int j = 1; j <= m; j++) {
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f[i] = static_cast<long long>(f[i]) * c[n - i + a[j] - 1][n - i - 1] % mod;
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}
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f[i] = static_cast<long long>(f[i]) * c[n][i] % mod;
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}
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for (int i = 0; i < n; i++) {
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if (i % 2 == 0) {
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ans = (static_cast<long long>(ans) + f[i]) % mod;
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} else {
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ans = (static_cast<long long>(ans) - f[i] + mod) % mod;
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}
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}
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cout << ans << endl;
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return 0;
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}
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