#include <bits/stdc++.h>
using namespace std;
const int MAXN = 400100;
vector<int> grafo[MAXN];
int a[MAXN], b[MAXN];
pair<int, int> p[MAXN];
int q[MAXN];
bool vis[MAXN];
int n;
vector<int> vec;
void dfs(int x) {
  vis[x] = true;
  vec.push_back(x);
  if (x > n)
    vis[x - n] = true;
  else
    vis[x + n] = true;
  for (int i = 0; i < grafo[x].size(); i++) {
    int at = grafo[x][i];
    if (vis[at] == true) continue;
    dfs(at);
  }
}
int main() {
  int t;
  cin >> t;
  while (t--) {
    cin >> n;
    vec.clear();
    for (int i = 1; i <= 2 * n; i++) {
      p[i] = {0, 0};
      grafo[i].clear();
      q[i] = 0;
      vis[i] = false;
      a[i] = b[i] = 0;
    }
    for (int i = 1; i <= n; i++) {
      int x;
      cin >> x;
      a[x] = i;
      q[x]++;
      if (p[x].first == 0)
        p[x].first = i;
      else
        p[x].second = i;
    }
    for (int i = 1; i <= n; i++) {
      int x;
      cin >> x;
      b[x] = i;
      q[x]++;
      if (p[x].first == 0)
        p[x].first = i;
      else
        p[x].second = i;
    }
    bool f = true;
    for (int i = 1; i <= n; i++)
      if (q[i] != 2) f = false;
    if (f == false) {
      cout << -1 << endl;
      continue;
    }
    for (int i = 1; i <= n; i++) {
      int p1 = p[i].first, p2 = p[i].second;
      if (a[i] > 0 && b[i] > 0) {
        grafo[p1].push_back(p2);
        grafo[p2].push_back(p1);
        grafo[p1 + n].push_back(p2 + n);
        grafo[p2 + n].push_back(p1 + n);
      } else {
        grafo[p1].push_back(p2 + n);
        grafo[p2].push_back(p1 + n);
        grafo[p2 + n].push_back(p1);
        grafo[p1 + n].push_back(p2);
      }
    }
    vector<int> resp;
    for (int i = 1; i <= n; i++) {
      if (vis[i] == true) continue;
      vec.clear();
      dfs(i);
      int h = vec.size();
      int cnt = 0;
      for (int j = 0; j < vec.size(); j++) {
        int at = vec[j];
        if (at > n) cnt++;
      }
      if (cnt < h - cnt) {
        for (int j = 0; j < vec.size(); j++)
          if (vec[j] > n) resp.push_back(vec[j] - n);
      } else {
        for (int j = 0; j < vec.size(); j++)
          if (vec[j] <= n) resp.push_back(vec[j]);
      }
    }
    cout << resp.size() << endl;
    for (int i = 0; i < resp.size(); i++) cout << resp[i] << " ";
    cout << endl;
  }
}
