cpp: solve «2092. Find All People With Secret»

Signed-off-by: Matej Focko <mfocko@redhat.com>

cpp/find-all-people-with-secret.cpp

@@ -1,51 +1,88 @@
 #include <algorithm>
-#include <iterator>
-#include <set>
+#include <map>
+#include <queue>
+#include <unordered_map>
+#include <unordered_set>
 #include <vector>
 
 class Solution {
-    void handle_previous_meeting(std::set<int> &overall,
-                                 std::set<int> &current) {
-        std::vector<int> intersection;
-        std::set_intersection(current.begin(), current.end(), overall.begin(),
-                              overall.end(), std::back_inserter(intersection));
-
-        if (!intersection.empty()) {
-            overall.insert(current.begin(), current.end());
+    void process_timeslot(std::vector<bool> &knows,
+                          const std::vector<std::pair<int, int>> &meetings) {
+        // Reconstruct the components for the same timeslot
+        // and also construct the source of secret
+        std::unordered_set<int> sources;
+        std::unordered_map<int, std::vector<int>> neighbors;
+        for (const auto &[a, b] : meetings) {
+            if (knows[a]) {
+                sources.insert(a);
+            }
+            if (knows[b]) {
+                sources.insert(b);
             }
 
-        current = {};
+            neighbors[a].push_back(b);
+            neighbors[b].push_back(a);
+        }
+
+        // Constructing queue from iterators is allowed since C++23…
+        std::queue<int> q;
+        for (const auto &p : sources) {
+            q.push(p);
+        }
+
+        // Classic BFS
+        while (!q.empty()) {
+            int u = q.front();
+            q.pop();
+
+            for (const auto &neighbor : neighbors[u]) {
+                if (knows[neighbor]) {
+                    continue;
+                }
+
+                knows[neighbor] = true;
+                q.push(neighbor);
+            }
+        }
     }
 
   public:
     std::vector<int> findAllPeople(int n,
                                    std::vector<std::vector<int>> meetings,
                                    int firstPerson) {
+        // Sort the meetings by time
         std::sort(meetings.begin(), meetings.end(),
                   [](const auto &l, const auto &r) { return l[2] < r[2]; });
 
-        std::set<int> knowers{0, firstPerson};
-
-        std::set<int> current_meeting{};
-        int timestamp = -1;
+        // Group meetings first
+        std::map<int, std::vector<std::pair<int, int>>> groups;
         for (const auto &meeting : meetings) {
             auto a = meeting[0];
             auto b = meeting[1];
             auto t = meeting[2];
 
-            if (t != timestamp) {
-                // handle the previous meeting
-                handle_previous_meeting(knowers, current_meeting);
-                timestamp = t;
+            groups[t].emplace_back(a, b);
         }
 
-            // just add the members and carry on
-            current_meeting.insert(a);
-            current_meeting.insert(b);
-        }
-        handle_previous_meeting(knowers, current_meeting);
+        // Keep “knowers” in the bitset
+        std::vector<bool> knows(n, false);
+        knows[0] = true;
+        knows[firstPerson] = true;
 
-        return std::vector<int>(knowers.begin(), knowers.end());
+        // Go through the groups
+        for (const auto &[time, meetings] : groups) {
+            process_timeslot(knows, meetings);
+        }
+
+        // Reconstruct the list of people that know
+        std::vector<int> all_people;
+        for (auto i = 0; i < n; ++i) {
+            if (knows[i]) {
+                all_people.push_back(i);
+            }
+        }
+
+        return all_people;
     }
 };