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  1. #include<bits/stdc++.h>
  2. #include <vector>
  3. #include <ctime>
  4. #define ll long long
  5. #define mod 1000000007
  6. using namespace std;
  7. ll fact[100011],invfact[100011];
  8.  
  9. ll pow(ll a, ll b)
  10. {
  11.     ll x=1,y=a;
  12.     while(b > 0)
  13.     {
  14.         if(b%2 == 1)
  15.         {
  16.             x=(x*y);
  17.             if(x>mod) x%=mod;
  18.         }
  19.         y = (y*y);
  20.         if(y>mod) y%=mod;
  21.         b /= 2;
  22.     }
  23.     return x;
  24. }
  25. void preprocess()
  26. {
  27.     fact[0]=fact[1]=1;
  28.     invfact[0]=invfact[1]=-1;
  29.     for(int i=2;i<100001;i++)
  30.     {
  31.         fact[i]=(fact[i-1]*i);
  32.         fact[i]%=mod;
  33.         invfact[i]=-1;
  34.     }
  35.  
  36. }
  37. ll comb(ll n, ll k)
  38. {
  39.     if(invfact[k]==-1)
  40.     {
  41.         invfact[k]=pow(fact[k],mod-2);
  42.         invfact[k]%=mod;
  43.     }
  44.     if(invfact[n-k]==-1)
  45.     {
  46.         invfact[n-k]=pow(fact[n-k],mod-2);
  47.         invfact[n-k]%=mod;
  48.     }
  49.     return ( ( ( fact[n] * invfact[k] ) % mod ) * invfact[n-k] ) % mod;
  50. }
  51. int main()
  52.    {
  53.        preprocess();
  54.        cout<<comb(10,2);
  55.      }
  56.  
Success #stdin #stdout 0s 4972KB
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https://ideone.com/SWPmty
language:
C++14 (gcc 8.3)
created:
7 years ago
visibility:
public

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