【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+;
 int curL = , curR = , n, m, a[maxn], answer = , bl, cnt[maxn], ans[maxn];
 struct query{
     int l, r, p;
 }q[maxn];
 inline int read()
 {
     int ret=;
     char c=getchar();
     while (c<'' || c>'') c=getchar();
     while (c>='' && c<=''){
         ret=((ret<<)+(ret<<))+c-'';
         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]] ==  && answer==a[pos])
     {
         while(cnt[answer]!=) answer++;
     }
 }
 inline void remove(int pos)
 {
     cnt[a[pos]]--;
     if (cnt[a[pos]]==) answer=min(answer,a[pos]);
 }
 int main()
 {
     n = read(); m = read();
     bl = sqrt(n);

     for(int i = ; i <= n; i++)
     a[i] = read();

     for(int i = ; i <= m; i++)
     {
         q[i].l = read(); q[i].r = read();
         q[i].p = i;
     }
     sort(q+,q++m,cmp);
     for(int i = ; 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 = ; i <= m; i++)
     printf("%d\n",ans[i]);
     return ;
 }