Submission #21295

#	TimeUTC-0	Username	Problem	Language	Result	Execution time	Memory
21295	2017-04-12 14:24:42	suzy	순열 (kriii4_T)	C++14	100 / 100	369 ms	8928 KiB

T

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#include <stdio.h>
#define MOD 1000000007
typedef long long lli;
lli fac[1000002];
lli pw(lli n, lli x) {
  if(x==0) return 1;
  if(x%2) return n*pw(n,x-1)%MOD;
  else { lli ret=pw(n,x/2); return ret*ret%MOD; }
}
int main() {
  lli n, k;
  fac[0]=1;
  for(lli i=1;i<=1000001;i++)
    fac[i]=fac[i-1]*i%MOD;
  scanf("%lld %lld",&n,&k); k++;
  if(k>n) {
    puts("0"); return 0;
  }
  lli res=0, ncl=fac[n+1]*pw(fac[n-k],MOD-2)%MOD*pw(fac[k+1],MOD-2)%MOD;
  for(lli l=k;l<=n;l++) {
    res+=(n-l+1)*fac[l]%MOD*fac[n-l]%MOD*ncl%MOD;
    res%=MOD;
    ncl=ncl*(n-l)%MOD*pw(l+2,MOD-2)%MOD;
  }
  printf("%lld\n",res);
  return 0;
 
 
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Compilation message (stderr)

T.cpp: In function 'int main()':
T.cpp:19:26: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%lld %lld",&n,&k); k++;
                          ^

• Subtask 1: 8 / 8
• Subtask 2: 92 / 92