by hansang:

#include<bits/stdc++.h>
using namespace std;
const int N=1e6+10;
map<int, int> mp;
int a[N];
int main(){
	int n, K; scanf("%d%d", &n, &K);
	for(int i=1; i<=n; i++) scanf("%d", &a[i]);
	int l=1, r=0, sum=0, ans=0; K++;
	while(r<n){
		if(++mp[a[++r]]==1) sum++;
		while(sum>K){
			if(--mp[a[l++]]==0) sum--;
		} 
		ans=max(ans, mp[a[r]]);
	}
	printf("%d\n", ans);
	return 0;
}

by hansang:

#include<bits/stdc++.h>
using namespace std;
const int N=1e6+10;
map<int, int> mp;
int a[N];
int main(){
	int n, K; scanf("%d%d", &n, &K);
	for(int i=1; i<=n; i++) scanf("%d", &a[i]);
	int l=1, r=0, sum=0, ans=0; K++;
	while(r<n){
		if(++mp[a[++r]]==1) sum++;
		while(sum>K){
			if(--mp[a[l++]]==0) sum--;
		} 
		ans=max(ans, mp[a[r]]);
	}
	printf("%d\n", ans);
	return 0;
}