bzoj3585: mex

传送门：http://www.lydsy.com:808/JudgeOnline/problem.php?id=3585

思路：又是一道莫队+分块。

对权值分块，大于n的直接不用管。

#include<cmath>
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
const int maxn=200010,maxb=450;
using namespace std;
struct que{int l,r,id;}q[maxn];
int n,m,sz,cnt,a[maxn],s[maxn],bel[maxn],l[maxb],r[maxb],bloans[maxn],ans[maxn];
bool cmp(que a,que b){return bel[a.l]==bel[b.l]?a.r<b.r:bel[a.l]<bel[b.l];}

void add(int x){
	if (x>n) return;
	if (!s[x]++) bloans[bel[x]]++;
}

void del(int x){
	if (x>n) return;
	if (!--s[x]) bloans[bel[x]]--;
}

int query(){
	int i;if (!s[0]) return 0;
	for (i=1;i<=cnt;i++)
		if (bloans[i]!=r[i]-l[i]+1) break;
	for (int j=l[i];j<=r[i];j++)
		if (!s[j]) return j;
}

void work(){
	for (int i=1,l=1,r=0;i<=m;i++){
		for (;r<q[i].r;r++) add(a[r+1]);
		for (;r>q[i].r;r--) del(a[r]);
		for (;l<q[i].l;l++) del(a[l]);
		for (;l>q[i].l;l--) add(a[l-1]);
		ans[q[i].id]=query();
	}
	for (int i=1;i<=m;i++) printf("%d\n",ans[i]);
}

int main(){
	scanf("%d%d",&n,&m),sz=sqrt(n),cnt=n/sz+(n%sz>0);
	for (int i=1;i<=n;i++) bel[i]=(i-1)/sz+1;
	for (int i=1;i<=n;i++){r[bel[i]]=i;if (!l[bel[i]]) l[bel[i]]=i;}
	for (int i=1;i<=n;i++) scanf("%d",&a[i]),a[i];
	for (int i=1;i<=m;i++) scanf("%d%d",&q[i].l,&q[i].r),q[i].id=i;
	sort(q+1,q+1+m,cmp),work();
	return 0;
}