题目：
There is a game called “Unique Bid Auction”. You can read more about it here: https://en.wikipedia.org/wiki/Unique_bid_auction (though you don’t have to do it to solve this problem).

Let’s simplify this game a bit. Formally, there are n participants, the i-th participant chose the number ai. The winner of the game is such a participant that the number he chose is unique (i. e. nobody else chose this number except him) and is minimal (i. e. among all unique values of a the minimum one is the winning one).

Your task is to find the index of the participant who won the game (or -1 if there is no winner). Indexing is 1-based, i. e. the participants are numbered from 1 to n.

You have to answer t independent test cases.

Input
The first line of the input contains one integer t (1≤t≤2⋅104) — the number of test cases. Then t test cases follow.

The first line of the test case contains one integer n (1≤n≤2⋅105) — the number of participants. The second line of the test case contains n integers a1,a2,…,an (1≤ai≤n), where ai is the i-th participant chosen number.

It is guaranteed that the sum of n does not exceed 2⋅105 (∑n≤2⋅105).

Output
For each test case, print the answer — the index of the participant who won the game (or -1 if there is no winner). Note that the answer is always unique.

Example
inputCopy
6
2
1 1
3
2 1 3
4
2 2 2 3
1
1
5
2 3 2 4 2
6
1 1 5 5 4 4
outputCopy
-1
2
4
1
2
-1
题解：
输入的时候计数，如果只计为1时候比较大小并且记录位置即可
上代码！

#include <bits/stdc++.h>
using namespace std;
int a[2000000];
int b[2000000];
int main()
{
  int t;
  cin>>t;
  while(t--){
    int n;
    scanf("%d",&n);
    memset(b,0,sizeof(int)*220000);
    for(long long i=0;i<n;i++){
      scanf("%d",&a[i]);
      b[a[i]]++;
    }
    long long ma=300000;
    int ret=0;
    int flag=0;
    for(int i=0;i<n;i++){
      if(b[a[i]]==1){
        if(a[i]<ma){
          ma=a[i];
          ret=i+1;
          flag=1;
        }
      }
    }
    if(flag==0) printf("-1\n");
      else printf("%d\n",ret);
  }
  return 0;
}