RT，对于下面的这份代码

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll N = 500010;
ll readL(){
	ll x = 0;
	char ch = getchar();
	while(!('0'<=ch&&ch<='9')) ch = getchar();
	x = ch-'0';
	ch = getchar();
	while('0'<=ch&&ch<='9'){
		x = x*10+ch-'0';
		ch = getchar();
	}
	return x;
}
char readC(){
	char ch;
	ch = getchar();
	while(!('A'<=ch&&ch<='Z')) ch = getchar();
	return ch;
}
ll n,m;
ll a[N],res[N],BlockSize,cnt[1000010],tot[1000010];\
ll b[N],c[N];
ll ans = 0;
#define bel(x) (x/BlockSize)
struct UPD{
	ll idx,val;
} cha[N];
struct QUS{
	ll ql,qr,t,id;
	bool operator <(const QUS &b) const{
		if(bel(ql)!=bel(b.ql)) return bel(ql)<bel(b.ql);
		if(bel(qr)!=bel(b.qr)) return bel(qr)<bel(b.qr);
		return t<b.t;
	}
} Q[N];
void add(ll val){
	tot[cnt[val]]--;
	cnt[val]++;
	tot[cnt[val]]++;
}
void del(ll val){
	tot[cnt[val]]--;
	cnt[val]--;
	if(cnt[val]<0){//注意这里
		cout << val <<endl;
		while(1);
	}
	tot[cnt[val]]++;
}
void tchg(ll cpos,ll qid){
	if(cha[cpos].idx>=Q[qid].ql&&cha[cpos].idx<=Q[qid].qr){
		del(a[cha[cpos].idx]);
		add(cha[cpos].val);
	}
	swap(a[cha[cpos].idx],cha[cpos].val);
}
int main(){
	n = readL();m = readL();
	BlockSize = (double)pow(n,0.666);
	ll totU = 0,totQ = 0,blen = 0;
	for(ll i = 1; i <= n; i++){a[i] = readL();b[++blen] = a[i];}
	for(ll i = 1; i <= m; i++){
		ll op = readL();
		if(op==2){
			totU++;
			cha[totU].idx = readL();
			cha[totU].val = readL();
			b[++blen] = cha[totU].val;
		} else {
			totQ++;
			Q[totQ] = (QUS){readL(),readL(),totU,totQ};
		}
	}
	sort(b+1,b+blen+1);
	ll len = unique(b+1,b+blen+1)-b-1;
	for(ll i = 1; i <= n; i++){
		//ll x = a[i];
		a[i] = lower_bound(b+1,b+len+1,a[i])-b;
		//c[a[i]] = x;
	}
	for(ll i = 1; i <= totU; i++){
		//ll x = cha[i].val;
		cha[i].val = lower_bound(b+1,b+len+1,cha[i].val)-b;
		//c[a[i]] = x;
	}
	sort(Q+1,Q+totQ+1);
	for(ll i = 1,l = 1,r = 0,cur = 0; i <= totQ; i++){
		while(l<Q[i].ql) del(a[l++]);
        while(l>Q[i].ql) add(a[--l]);
        while(r<Q[i].qr) add(a[++r]);
        while(r>Q[i].qr) del(a[r--]);
        while(cur<Q[i].t) tchg(++cur,i); 
        while(cur>Q[i].t) tchg(cur--,i);
        for(res[Q[i].id] = 1; tot[res[Q[i].id]] > 0; ++res[Q[i].id]);
	}
	for(ll i = 1; i <= totQ; i++) printf("%lld\n",res[i]);
	return 0;
}

通过 del 里的特判，当一个数的出现次数成为负数时直接让程序 T 掉，这显然是不可能的，可是程序 TLE on #2，但是将莫队移动端点的

	while(l<Q[i].ql) del(a[l++]);
	while(l>Q[i].ql) add(a[--l]);
	while(r<Q[i].qr) add(a[++r]);
	while(r>Q[i].qr) del(a[r--]);

改成

	while(l>Q[i].ql) add(a[--l]);
	while(r<Q[i].qr) add(a[++r]);
	while(l<Q[i].ql) del(a[l++]);
	while(r>Q[i].qr) del(a[r--]);

就 ac 了，为什么会这样？