组合数学——cf1065E

从两端到中间分段，然后累乘即可

#include<bits/stdc++.h>
using namespace std;
#define mod 998244353
#define maxn 200005
#define ll long long 
ll n,m,A,b[maxn];

ll Pow(ll a,ll b){
    ll res=1;
    while(b){
        if(b%2)res=a*res%mod;
        b>>=1;a=a*a%mod;
    }
    return res;
}

int main(){
    cin>>n>>m>>A;
    for(int i=1;i<=m;i++)cin>>b[i];
    sort(b+1,b+1+m);

    ll ans=Pow(A,n-2*b[m]);
    for(int i=1;i<=m;i++){
        ll p=Pow(A,b[i]-b[i-1]);
        ans=ans*((p*(p+1)/2)%mod)%mod;
    }
    cout<<ans<<endl;
}