【洛谷P3600】 随机数生成器

// luogu3600
#include<algorithm>
#include<iostream>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<cmath>
#define LL long long
#define Pi acos(-1.0)
#define free(a) freopen(a".in","r",stdin),freopen(a".out","w",stdout);
using namespace std;

const int maxn=2010,MOD=666623333;
int n,m,X,Q;
LL f[maxn],s[maxn],inv[maxn],ans;
struct data {int l,r;}q[maxn],t[maxn];

bool cmp(data a,data b) {
	return a.l==b.l ? a.r<b.r : a.l<b.l;
}
LL power(LL a,LL b) {
	LL res=1;
	while (b) {
		if (b&1) (res*=a)%=MOD;
		b>>=1;(a*=a)%=MOD;
	}
	return res;
}
int main() {
	scanf("%d%d%de",&n,&X,&Q);
	for (int i=1;i<=Q;i++) scanf("%d%d",&q[i].l,&q[i].r);
	sort(q+1,q+1+Q,cmp);
	for (int i=1;i<=Q;i++) {
		while (q[m].r>=q[i].r && m) m--;
		if (q[i].l>q[m].l && q[i].r>q[m].r) q[++m]=q[i];
	}
	for (int l=1,r=0,i=1;i<=n;i++) {
		while (r<m && q[r+1].l<=i && i<=q[r+1].r) r++;
		while (l<=m && q[l].r<i) l++;
		t[i]=(data){l,r};
	}
	f[0]=inv[0]=1;
	for (int x=1;x<=X;x++) {
		LL T=power(X,MOD-2)*(x-1)%MOD,P=(1-T+MOD)%MOD,B=power(P,MOD-2),K=1,S=1,sum=0;
		for (int i=1;i<=n;i++) inv[i]=inv[i-1]*B%MOD;
		for (int p=0,i=1;i<=n;i++,(K*=P)%=MOD) {
			for (;p<i && t[p].r<t[i].l-1;p++) (S+=MOD-inv[p]*f[p]%MOD)%=MOD;
			f[i]=S*K%MOD*T%MOD;
			(S+=f[i]*inv[i]%MOD)%=MOD;
		}
		K=1;
		for (int i=n;i>=1;i--,(K*=P)%=MOD) if (t[i].r==m) (sum+=f[i]*K%MOD)%=MOD;
		(ans+=(1-sum+MOD))%=MOD;
	}
	printf("%lld",ans);
	return 0;
}