1714B-RemovePrefix.cpp

#include <cstdio>
#include <vector>
#include <set>

int main(){

    long t; scanf("%ld", &t);
    while(t--){
        long n; scanf("%ld", &n);
        std::vector<long> a(n); for(long p = 0; p < n; p++){scanf("%ld", &a[p]);}
        std::set<long> s; long res(0);
        for(long p = n - 1; p >= 0; p--){
            if(s.count(a[p])){res = p + 1; break;}
            s.insert(a[p]);
        }

        printf("%ld\n", res);
    }

}