https://pintia.cn/problem-sets/994805342720868352/problems/994805354762715136
#include<bits/stdc++.h>
using namespace std;
const int N=1e5+10;
int a[N],n;
int main(void)
{
cin>>n;
for(int i=0;i<n;i++) cin>>a[i];
sort(a,a+n);
for(int i=n;i>=0;i--)
{
int len=upper_bound(a,a+n,i)-a;
len=(n-1)-len+1;
if(len>=i)
{
cout<<i;
return 0;
}
}
return 0;
}