A bunch new problems

Codeforces Round 1065 (Div. 3)/F. Rae Taylor and Trees (hard version).cpp

@@ -93,42 +93,52 @@ void solve()
     vi p(n);
     cin >> p;
 
+    vi suf(n + 1);
     vi pos(n);
-    rep(i, n) {
+    for (int i = n - 1; i >= 0; i--) {
+        suf[i] = max(suf[i + 1], p[i]);
         pos[p[i] - 1] = i;
     }
 
-    stack<pair<int, int>> s;
+    V<pair<int, int>> ans;
 
-    pair<int, int> big = {0, 0};
-
-    V<pair<int, int>> edges;
-
-    for (int i = n - 1; i >= 0; i--) {
-        while (!s.empty() && s.top().first > pos[i]) {
-            edges.emplace_back(s.top().second + 1, i + 1);
-            s.pop();
+    int now = 1;
+    int lim = n;
+    int ma = 1;
+    while (now < n) {
+        int ps = pos[now - 1];
+        if (now > ma) {
+            cout << "No\n";
+            return;
         }
 
-        if (big.first > pos[i]) {
-            edges.emplace_back(big.second + 1, i + 1);
+        if (ps >= lim) {
+            now++;
             continue;
         }
 
-        big = {pos[i], i};
-        s.emplace(pos[i], i);
+        nrep(i, ps + 1, n) {
+            if (p[i] < now) {
+                break;
+            }
+
+            ans.emplace_back(p[i], now);
+        }
+        if (now != ma) {
+            ans.emplace_back(ma, now);
+        }
+
+        lim = ps;
+        rmax(ma, suf[ps]);
     }
 
-    sortv(edges);
-    edges.erase(unique(all(edges)), edges.end());
-
-    if (edges.size() != n - 1) {
+    if (ans.size() < n - 1) {
         cout << "No\n";
         return;
     }
 
     cout << "Yes\n";
-    repv(i, edges) {
+    repv(i, ans) {
         cout << i.first << ' ' << i.second << '\n';
     }
 }