【luogu P3709 大爷的字符串题】 题解

题目链接：https://www.luogu.org/problemnew/show/P3709
离散化+区间众数..？

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
using namespace std;
const int maxn = 500000+10;
inline int read()
{
    int k=0;
    char c;
    c=getchar();
    while(!isdigit(c))c=getchar();
    while(isdigit(c)){k=(k<<3)+(k<<1)+c-'0';c=getchar();}
    return k;
}
int n, m, a[maxn], ans[maxn], cnt[maxn], bl, curL = 1, curR = 0, answer = 0, b[maxn], tot, sum[maxn];
struct query{
	int l, r, p;
}e[maxn];
bool cmp(query a, query b)
{
	return (a.l/bl) == (b.l/bl) ? a.r < b.r : a.l < b.l; 
}
void add(int pos)
{
	sum[cnt[a[pos]]]--;
	cnt[a[pos]]++;
	sum[cnt[a[pos]]]++;
	answer = max(answer, cnt[a[pos]]);
}
void remove(int pos)
{
	sum[cnt[a[pos]]]--;
	cnt[a[pos]]--;
	sum[cnt[a[pos]]]++;
	while(!sum[answer]) answer--;
}
int main()
{
	n = read(); m = read();
	
	bl = sqrt(n);
	
	for(int i = 1; i <= n; i++)
	a[i] = b[++tot] = read();
	
	sort(b+1, b+1+tot);
	tot = unique(b+1, b+1+tot)-b-1;
	for(int i = 1; i <= n; i++)
	a[i] = lower_bound(b+1,b+1+tot,a[i])-b;//离散化 
	
	for(int i = 1; i <= m; i++)
	{
		e[i].l = read(); e[i].r = read(); e[i].p = i;
	}
	
	sort(e+1, e+1+m, cmp);

	for(int i = 1; i <= m; i++)
	{
		int L = e[i].l, R = e[i].r;
		while(curL < L) remove(curL++);
		while(curL > L) add(--curL);
		while(curR < R) add(++curR);
		while(curR > R) remove(curR--);
		ans[e[i].p] = answer;
	}
	for(int i = 1; i <= m; i++)
	printf("%d
",0-ans[i]);
	return 0;
}
/*
7 5
52 1 52 52 1000000000 1000000000 1000000000
1 3
1 2
1 6
2 5
2 2
*/