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Codeforces Round 874 (Div. 3)/G. Ksyusha and Chinchilla.cpp

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+/* Problem URL: https://codeforces.com/problemset/problem/1833/G */
+
+#include <bits/stdc++.h>
+#include <ext/pb_ds/assoc_container.hpp>
+#include <ext/pb_ds/tree_policy.hpp>
+
+using namespace std;
+using namespace __gnu_pbds;
+
+template <class T, class comp = less<>>
+using ordered_set = tree<T, null_type , comp , rb_tree_tag , tree_order_statistics_node_update>;
+
+#define V vector
+
+#define rmin(a, b) a = min(a, b)
+#define rmax(a, b) a = max(a, b)
+
+#define rep(i, lim) for (int i = 0; i < (lim); i++)
+#define nrep(i, s, lim) for (int i = s; i < (lim); i++)
+
+#define repv(i, v) for (auto &i : (v))
+#define fillv(v) for (auto &itr_ : (v)) { cin >> itr_; }
+#define sortv(v) sort(v.begin(), v.end())
+#define all(v) (v).begin(), (v).end()
+
+using vi = vector<int>;
+using vvi = vector<vi>;
+using vvvi = vector<vvi>;
+using vvvvi = vector<vvvi>;
+
+using ll = long long;
+
+using vl = vector<ll>;
+using vvl = vector<vl>;
+using vvvl = vector<vvl>;
+using vvvvl = vector<vvvl>;
+
+template<class v>
+auto operator<<(ostream &os, const vector<v> &vec)->ostream& {
+    os << vec[0];
+    for (size_t i = 1; i < vec.size(); i++) {
+        os << ' ' << vec[i];
+    }
+    os << '\n';
+    return os;
+}
+
+template<class v>
+auto operator>>(istream &is, vector<v> &vec)->istream& {
+    for (auto &i : vec) {
+        is >> i;
+    }
+    return is;
+}
+
+template<class v>
+auto operator<<(ostream &os, const vector<vector<v>> &vec)->ostream& {
+    for (auto &i : vec) {
+        os << i[0];
+        for (size_t j = 1; j < i.size(); j++) {
+            os << ' ' << i[j];
+        }
+        os << '\n';
+    }
+    return os;
+}
+
+template<class v>
+auto operator>>(istream  &is, vector<vector<v>> &vec)->istream& {
+    for (auto &i : vec) {
+        for (auto &j : i) {
+            is >> j;
+        }
+    }
+    return is;
+}
+
+const int oo = INT32_MAX >> 1;
+const ll OO = INT64_MAX >> 1;
+
+
+void pre()
+{
+
+}
+
+#define TEST 1
+
+void solve()
+{
+    int n;
+    cin >> n;
+
+    V<set<pair<int, int>>> graph(n);
+
+    nrep(i, 1, n) {
+        int a, b;
+        cin >> a >> b;
+        a--, b--;
+        graph[a].emplace(b, i);
+        graph[b].emplace(a, i);
+    }
+
+    set<int> er;
+
+    bool pos = true;
+    function<int(int, int)> dfs = [&](int i, int p) {
+        int c = 0;
+        auto cur = graph[i].begin();
+        while (cur != graph[i].end()) {
+            auto [j, ind] = *cur;
+            graph[i].erase({j, ind});
+            cur = graph[i].begin();
+
+            if (graph[j].size() != 1) {
+                graph[j].erase({i, ind});
+                int res = dfs(j, ind);
+                if (res == 0) {
+                    c++;
+                    continue;
+                }
+
+                if (res == 1) {
+                    if (c != 0) {
+                        pos = false;
+                        return 2;
+                    }
+
+                    for (auto [k, _] : graph[i]) {
+                        er.insert(_);
+                        graph[k].erase({i, _});
+                        if (dfs(k, _) != 2) {
+                            pos = false;
+                            return 0;
+                        }
+                    }
+
+                    return 2;
+                }
+
+                er.insert(ind);
+                continue;
+            }
+
+            c++;
+        }
+
+        if (c > 2) {
+            pos = false;
+            return 0;
+        }
+
+        return c;
+    };
+
+    if (dfs(0, -1) != 2 || !pos) {
+        cout << "-1\n";
+        return;
+    }
+
+    er.erase(0);
+
+    cout << er.size() << '\n';
+    repv(i, er) {
+        cout << i << ' ';
+    }
+    cout << '\n';
+}
+
+int main()
+{
+    ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    pre();
+
+    int t;
+    (TEST && cin >> t) || (t = 1);
+    while (t--) {
+        solve();
+    }
+}