POJ-3320-Jessica's Reading Problem

#include<cstdio>
#include<cstring>
#include<iostream>
#include<set>
#include<map>
using namespace std;
const int maxn=1e6+1000;
int n,a[maxn],ans;
set<int> s;
map<int,int> f;
int main()
{
while(scanf("%d",&n)!=EOF)
{
s.clear();
f.clear();
for(int i=0;i<n;i++)
{
scanf("%d",&a[i]);
s.insert(a[i]);
}
int cnt=s.size(),pre=0,last=0,now=0;
ans=n;
while(1)
{
while(last<n&&now<cnt)
{
if(!f[a[last]])
now++;
f[a[last++]]++;
}
while(f[a[pre]]>1)
f[a[pre++]]--;
if(now==cnt)
{
ans=min(ans,last-pre);
f[a[pre++]]--;
now--;
}
if(last==n)
break;
}
printf("%d\n",ans);
}
return 0;
}