28 lines
646 B
C++
28 lines
646 B
C++
#include <bits/stdc++.h>
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//https://www.luogu.com.cn/blog/x4Cx58x54/solution-p2638
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//经典题
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using namespace std;
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typedef unsigned long long ULL;
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int n, a, B;
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ULL ans;
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//求组合数的优化后办法
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//优点：从1开始除和乘，可以防止过早溢出和除法除不尽
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ULL C(int n, int m) {
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ULL sum = 1;
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for (int i = 1; i <= m; i++)
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sum = sum * (n - m + i) / i;
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return sum;
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}
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int main() {
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cin >> n >> a >> b;
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for (int i = 0; i <= a; i++)
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for (int j = 0; j <= b; j++)
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ans += C(n + i - 1, n - 1) * C(n + j - 1, n - 1);
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cout << ans << endl;
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return 0;
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} |