【luogu P4137 Rmq Problem / mex】 题解

题目链接：https://www.luogu.org/problemnew/show/P4137

求区间内最大没出现过的自然数

在add时要先判断会不会对当前答案产生影响，如果有就去找下一个答案。

#include <cstdio>
#include <algorithm>
#include <iostream>
#include <cmath>
using namespace std;
const int maxn = 2e5+10;
int curL = 1, curR = 0, n, m, a[maxn], answer = 0, bl, cnt[maxn], ans[maxn];
struct query{
    int l, r, p;
}q[maxn];
inline int read()
{
    int ret=0;
    char c=getchar();
    while (c<'0' || c>'9') c=getchar();
    while (c>='0' && c<='9'){
        ret=((ret<<3)+(ret<<1))+c-'0';
        c=getchar();
    }
    return ret;
}
bool cmp(const query &a, const query &b)
{
    if(a.l / bl == b.l / bl) return a.r < b.r;
    else return a.l < b.l;
}
void add(int pos) 
{
    cnt[a[pos]]++;
    if(cnt[a[pos]] == 1 && answer==a[pos])
    {
        while(cnt[answer]!=0) answer++;
    }
}
inline void remove(int pos) 
{
    cnt[a[pos]]--;
    if (cnt[a[pos]]==0) answer=min(answer,a[pos]);
} 
int main()
{
    n = read(); m = read();
    bl = sqrt(n);
    
    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].p = i;
    }
    sort(q+1,q+1+m,cmp);
    for(int i = 1; i <= m; i++)
    {
        int L = q[i].l, R = q[i].r;
        while(curL < L) remove(curL++);
        while(curL > L)    add(--curL);
        while(curR < R)    add(++curR);
        while(curR > R) remove(curR--);
        ans[q[i].p] = answer;
    }
    for(int i = 1; i <= m; i++)
    printf("%d\n",ans[i]);
    return 0; 
}