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29 lines
595 B
29 lines
595 B
#include <bits/stdc++.h>
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using namespace std;
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typedef long long LL;
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const int p = 1e9 + 7;
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int n;
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int qmi(int a, int b) {
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int res = 1;
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while (b) {
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if (b & 1) res = (LL)res * a % p;
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b >>= 1;
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a = (LL)a * a % p;
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}
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return res;
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}
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int main() {
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cin >> n;
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int res = 1;
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// ① 分子,2n*(2n-1)*(2n-2)*...*(n+1)
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for (int i = 2 * n; i > n; i--) res = (LL)res * i % p;
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// ② 分母,inv(n+1)*inv(n)*inv(n-1)*...*inv(1) %p
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for (int i = n + 1; i; i--) res = (LL)res * qmi(i, p - 2) % p;
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// 输出
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cout << res << endl;
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return 0;
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} |