【BZOJ 3339 / BZOJ 3585 / luogu 4137】Rmq Problem / mex

#include<bits/stdc++.h>
using namespace std;
#define File "testdata"
#define ll long long
inline void file(){
    freopen(File".in","r",stdin);
    freopen(File".ans","w",stdout);
}
inline int read(){
    int x=0,f=1;   char ch=getchar();
    while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1; ch=getchar();}
    while(ch>=‘0‘&&ch<=‘9‘){x=(x<<1)+(x<<3)+ch-‘0‘; ch=getchar();}
    return x*f;
}
const int mxn = 2e5+5;
int n,m,Block;
int a[mxn],cnt[mxn],ans[mxn];
struct Q{
    int l,r,id;
}q[mxn];

inline bool cmp1(Q x,Q y){
    if((x.l/Block)^(y.l/Block)) return x.l < y.l;
    else if((x.l/Block)&1) return x.r < y.r;
    return x.r > y.r;
}

int answer = 0;
inline void mov1(int x){
    cnt[a[x]] ++;
    if(cnt[a[x]]==1 && answer==a[x]){
        int t = a[x];
        while(++t && !cnt[t]){
            answer = t;
            return;
        }
    }
}//add
inline void mov2(int x){
    cnt[a[x]] --;
    if(answer>a[x] && !cnt[a[x]]){
        answer = a[x];
        return;
    }
}//del

int main(){
//    file();
    n = read(),m = read();
    for(int i = 1;i <= n; ++i) a[i] = read();
    for(int i = 1;i <= m; ++i)
        q[i].l = read(),q[i].r = read(),q[i].id = i;
    Block = (int)sqrt(n);
    int l(1),r(0);
    sort(q+1,q+m+1,cmp1);
    for(int i = 1;i <= m; ++i){
        int ql = q[i].l,qr = q[i].r;
        while(ql < l) mov1(l-1),l--;
        while(ql > l) mov2(l),l++;
        while(qr > r) mov1(r+1),r++;
        while(qr < r) mov2(r),r--;
        ans[q[i].id] = answer;
    }
    for(int i = 1;i <= m; ++i) printf("%d\n",ans[i]);
    return 0;
}
/*
5 5
2 1 0 2 1
3 3
2 3
2 4
1 2
3 5
*/