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Sol

這題可能是假的
離線莫隊搞一搞，把數字再分塊搞一搞，就行了

# include <bits/stdc++.h>
# define IL inline
# define RG register
# define Fill(a, b) memset(a, b, sizeof(a))
using namespace std;
typedef long long ll;
const int _(2e5 + 5);

IL ll Input(){
    RG char c = getchar(); RG ll x = 0, z = 1;
    for(; c < '0'
 
 || c > '9'; c = getchar()) z = c == '-' ? -1 : 1;
    for(; c >= '0' && c <= '9'; c = getchar()) x = (x << 1) + (x << 3) + (c ^ 48);
    return x * z;
}

int n, q, bl[_], val[_], ans[_], cnt[_], sum[_];
struct Qry{
    int l, r, id;
    IL bool
 
 operator <(RG Qry B) const{
        return bl[l] != bl[B.l] ? bl[l] < bl[B.l] : r < B.r;
    }
} qry[_];

IL void Modify(RG int x, RG int d){
    if(d > 0){
        if(!cnt[x]) ++sum[x / 500];
        ++cnt[x];
    }
    else{
        --cnt[x];
        if(!cnt[x]) --sum[x / 500];
    }
}

IL int
 
 Calc(){
    RG int ret = 0;
    for(RG int i = 0; ; ++i)
        for(RG int j = 0; j < 500 && sum[i] != 500; ++j)
            if(!cnt[i * 500 + j]) return i * 500 + j;
}

int main(RG int argc, RG char* argv[]){
    n = Input(); q = Input();
    RG int blo = sqrt(n);
    for(RG int i = 1; i <= n; ++i){
        val[i] = Input();
        bl[i] = (i - 1) / blo + 1;
    }
    for(RG int i = 1; i <= q; ++i) qry[i] = (Qry){Input(), Input(), i};
    sort(qry + 1, qry + q + 1);
    for(RG int L = qry[1].l, R = qry[1].l - 1, i = 1; i <= q; ++i){
        while(L < qry[i].l) Modify(val[L], -1), ++L;
        while(L > qry[i].l) --L, Modify(val[L], 1);
        while(R < qry[i].r) ++R, Modify(val[R], 1);
        while(R > qry[i].r) Modify(val[R], -1), --R;
        ans[qry[i].id] = Calc();
    }
    for(RG int i = 1; i <= q; ++i) printf("%d\n", ans[i]);
    return 0;
}

Luogu4137：Rmq Problem/mex