题目链接：http://www.lightoj.com/volume_showproblem.php?problem=1005
纸上画一下，找了一下规律，Ank*Cnk.
1 //#pragma comment(linker, "/STACK:102400000, 102400000") 2 #include <algorithm> 3 #include <iostream> 4 #include <cstdlib> 5 #include <cstring> 6 #include <cstdio> 7 #include <vector> 8 #include <cmath> 9 #include <ctime> 10 #include <list> 11 #include <set> 12 #include <map> 13 using namespace std; 14 typedef long long LL; 15 typedef pair <int, int> P; 16 const int N = 1e5 + 5; 17 18 LL Anm(LL n, LL m) { 19 LL ans = 1; 20 for(LL i = n; i >= n - m + 1; --i) { 21 ans = ans * i; 22 } 23 return ans; 24 } 25 26 int main() 27 { 28 int t; 29 LL n, k; 30 scanf("%d", &t); 31 for(int ca = 1; ca <= t; ++ca) { 32 scanf("%lld %lld", &n, &k); 33 printf("Case %d: ", ca); 34 if(k > n) { 35 printf("0 "); 36 continue; 37 } 38 LL ans = Anm(n, k); 39 LL temp = ans; 40 for(LL i = k; i >= 2; --i) { 41 temp /= i; 42 } 43 printf("%lld ", ans * temp); 44 } 45 return 0; 46 }