CodeForces - 1445D（思维）

[题目链接](https://vjudge.net/problem/CodeForces-1445D) ------------ 仔细思考可以发现，对于${a}$，任意的$f(p,q)$值唯一 ------------ ```cpp #include <bits/stdc++.h> using namespace std; typedef long long ll; const int M=4e5+5,mod=998244353; int n,a[M]; ll fac[M],ans; ll qpow(ll a,int b){ll y=1;for(;b;b>>=1,a=a*a%mod)if(b&1)y=y*a%mod;return y;} int main() { scanf("%d",&n); fac[0]=1; for(int i=1;i<=2*n;i++) { scanf("%d",&a[i]); fac[i]=fac[i-1]*i%mod; } sort(a+1,a+1+2*n); for(int i=1;i<=n;i++)ans=(ans+a[i+n]-a[i])%mod; ll inv=qpow(fac[n],mod-2); ans=ans*fac[2*n]%mod*inv%mod*inv%mod; printf("%lld\n",ans); return 0; } ```