Submission #33870319

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
#define MOD 998244353
#define rep(i,a,n) for (ll i=a;i<(ll)n;i++)
#define per(i,a,n) for (ll i=n;i-->(ll)a;)
#define pb push_back

ll read(){ll r;scanf("%lld",&r);return r;} // read

int n;

int qpow(ll v,int pwr){
  ll r = 1;
  while(pwr){
    if(pwr%2) (r*=v)%=MOD;
    (v*=v)%=MOD;
    pwr/=2;
  }
  return r;
}

int fac[20000010] = {1};
int ifac[20000010];

ll binom(ll n,ll m){
  if(n < 0 || m < 0 || m > n) return 0;
  return fac[n]*(ll)ifac[m]%MOD*(ll)ifac[n-m]%MOD;
}

int main(){
  int n = read();
  rep(i,1,2*n+1) fac[i] = fac[i-1]*(ll)i % MOD;
  ifac[2*n] = qpow(fac[2*n],MOD-2);
  per(i,0,2*n) ifac[i] = ifac[i+1] * (ll)(i+1) % MOD;
  ll ans = 0;
  ll p3 = qpow(3,n-1);
  ll inv3_sq = qpow(3*3, MOD-2);
  // sum_{i=[0..2n]}binom(2n,i)binom(2n-i,n-1-2i) 3^{n-1-2i}
  rep(i,0,2*n+1) {
    if(n-1-2*i < 0) break;
    (ans += binom(2*n,i)*binom(2*n-i,n-1-2*i)%MOD*p3) %= MOD;
    (p3 *= inv3_sq)%=MOD;
  }
  printf("%lld\n",ans*qpow(n,MOD-2) % MOD); // 1/n
  return 0;
}

Submission Info

Compile Error

./Main.cpp: In function ‘ll read()’:
./Main.cpp:10:21: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result]
   10 | ll read(){ll r;scanf("%lld",&r);return r;} // read
      |                ~~~~~^~~~~~~~~~~

Judge Result