#include <bits/stdc++.h>
using namespace std;
using lli = long long int;
const int MOD = 1000000007;
const int MOD1 = 998244353;
const int maxn = 100010;
const int lim = (int)1e9;
vector<int> v[2 * maxn];
vector<int> adj[2 * maxn];
vector<int> parent;
vector<int> vis;
set<int> st;
int FIND(int node) {
  if (parent[node] < 0) return node;
  return parent[node] = FIND(parent[node]);
}
void DSU(int a, int b) {
  a = FIND(a);
  b = FIND(b);
  if (a != b) {
    if (parent[a] > parent[b]) swap(a, b);
    parent[a] += parent[b];
    parent[b] = a;
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t = 0, n = 0;
  cin >> t;
  while (t--) {
    cin >> n;
    parent.resize(n);
    vis.resize(n);
    for (int i = 0; i < n; ++i) {
      v[i].clear();
      adj[i].clear();
      parent[i] = -1;
      vis[i] = 0;
    }
    vector<int> a(n);
    vector<int> b(n);
    for (int i = 0; i < n; ++i) {
      cin >> a[i];
      --a[i];
      v[a[i]].push_back(i);
    }
    for (int i = 0; i < n; ++i) {
      cin >> b[i];
      --b[i];
      v[b[i]].push_back(n + i);
    }
    bool flag = false;
    for (int i = 0; i < n; ++i) {
      if ((int)v[i].size() != 2) {
        flag = true;
        break;
      }
      if (v[i][0] < n && v[i][1] >= n) DSU(v[i][0], v[i][1] - n);
    }
    if (flag) {
      cout << "-1" << endl;
      continue;
    }
    for (int i = 0; i < n; ++i) {
      if (v[i][0] >= n && v[i][1] >= n) {
        if (FIND(v[i][0] - n) == FIND(v[i][1] - n)) {
          flag = true;
          break;
        } else {
          adj[FIND(v[i][0] - n)].push_back(FIND(v[i][1] - n));
          adj[FIND(v[i][1] - n)].push_back(FIND(v[i][0] - n));
        }
      } else if (v[i][0] < n && v[i][1] < n) {
        if (FIND(v[i][0]) == FIND(v[i][1])) {
          flag = true;
          break;
        } else {
          adj[FIND(v[i][0])].push_back(FIND(v[i][1]));
          adj[FIND(v[i][1])].push_back(FIND(v[i][0]));
        }
      }
    }
    if (flag) {
      cout << "-1" << endl;
      continue;
    }
    st.clear();
    queue<int> q;
    for (int i = 0; i < n; ++i) {
      vector<int> parity[2];
      if (FIND(i) == i && vis[i] == 0) {
        q.push(i);
        vis[i] = 2;
        q.push(0);
      }
      int odd = 0, even = 0;
      while (!q.empty()) {
        int f = q.front();
        q.pop();
        int s = q.front();
        q.pop();
        if (s & 1) {
          odd -= parent[f];
          parity[1].push_back(f);
        } else {
          even -= parent[f];
          parity[0].push_back(f);
        }
        for (auto &j : adj[f]) {
          if (vis[j]) {
            if (vis[j] % 2 != (s + 1) % 2) {
              flag = true;
              break;
            }
            continue;
          }
          q.push(j);
          q.push(s + 1);
          if ((s + 1) & 1)
            vis[j] = 1;
          else
            vis[j] = 2;
        }
        if (flag) break;
      }
      if (flag) break;
      if (odd > even) {
        for (auto &j : parity[0]) st.insert(j);
      } else {
        for (auto &j : parity[1]) st.insert(j);
      }
    }
    if (flag) {
      cout << "-1" << endl;
      continue;
    }
    vector<int> res;
    for (int i = 0; i < n; ++i)
      if (st.count(FIND(i))) res.push_back(i + 1);
    cout << (int)res.size() << endl;
    for (auto &i : res) cout << i << " ";
    cout << endl;
  }
}
