Vaihdot

#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pii pair<int, int>
#define x first
#define y second

bool solve(int n, vector<int>& arr, vector<pii>& ans) {
  vector<int> pos(n);
  for (int i=0; i<n; i++)
    pos[arr[i]] = i;
  auto mv = [&](int i, int j) {
    if (abs(i-j)<=1 || arr[i]==i) return;
    swap(arr[i], arr[j]);
    swap(pos[arr[i]], pos[arr[j]]);
    ans.emplace_back(i, j);
  };
  for (int j=0; j<n; j++) {
    int x = (j+1)%n;
    mv(pos[x], x);
    mv(pos[x], 0);
    mv(pos[x], x);
    mv(pos[x], n-1);
    mv(pos[x], x);
    if (arr[x] != x)
      return false;
  }
  return true;
}

signed main() {
  ios::sync_with_stdio(0);
  int t;
  cin >> t;
  while (t--) {
    int n;
    cin >> n;
    vector<int> arr(n);
    for (int i=0; i<n; i++) {
      cin >> arr[i];
      arr[i]--;
    }
    vector<pii> ans;
    if (!solve(n, arr, ans))
      cout << "-1\n";
    else {
      cout << ans.size() << '\n';
      for (auto p : ans)
        cout << p.x+1 << ' ' << p.y+1 << '\n';
    }
  }
}

Test 1

Subtask: 1, 2

Verdict: ACCEPTED

Test 2

Subtask: 2

Verdict: ACCEPTED