题解 | mex

#include <iostream>
using namespace std;

#include<set>
#include<algorithm>
int main(){
    set<long long>a;
    int n;
    cin>>n;
    int t=0;
    int c=0;
while(n--){
    
    
long long b;
cin>>b;

if(b==0){
    t++;

}
if(b>0){
    a.insert(b);
}



}
c=a.size();
if(t==0){if(c>1)
    cout<<-1;
    else{
        cout<<0;
    }
}
else{if(!a.empty())
    cout<<*prev(a.end())-c+1;
    else{
        cout<<0;
    }
}



}