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50 lines
1.2 KiB
50 lines
1.2 KiB
#include <bits/stdc++.h>
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using namespace std;
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#define int long long
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#define endl "\n"
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const int N = 20, mod = 1e9 + 7;
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int A[N];
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int n, m;
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// 快速幂
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int qmi(int a, int k) {
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int res = 1;
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while (k) {
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if (k & 1) res = res * a % mod;
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a = a * a % mod;
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k >>= 1;
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}
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return res;
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}
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int C(int a, int b) {
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if (a < b) return 0;
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int up = 1, down = 1;
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for (int i = a; i > a - b; i--) up = i % mod * up % mod;
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for (int i = 1; i <= n - 1; i++) down = i * down % mod; //(n-1)! % mod
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down = qmi(down, mod - 2); // 费马小定理求逆元
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return up * down % mod; // 费马小定理
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}
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signed main() {
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cin >> n >> m;
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for (int i = 0; i < n; i++) cin >> A[i];
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int res = C(n + m - 1, n - 1);
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for (int i = 1; i < (1 << n); i++) {
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int sum = 0, cnt = 0;
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for (int j = 0; j < n; j++) {
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if (i >> j & 1) {
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sum += A[j] + 1;
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cnt++;
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}
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}
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if (cnt & 1)
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res = (res - C(m + n - 1 - sum, n - 1) + mod) % mod;
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else
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res = (res + C(m + n - 1 - sum, n - 1)) % mod;
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}
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cout << res << endl;
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} |