Create course-schedule-iv.cpp

Author: kamyu104

C++/course-schedule-iv.cpp

@@ -0,0 +1,62 @@
+// Time:  O(n^3)
+// Space: O(n^2)
+
+class Solution {
+public:
+    vector<bool> checkIfPrerequisite(int n, vector<vector<int>>& prerequisites, vector<vector<int>>& queries) {
+        const auto& reachable = floydWarshall(n, prerequisites);
+        vector<bool> result;
+        for (const auto& q : queries) {
+            result.emplace_back(reachable.count(q[0] * n + q[1]));
+        }
+        return result;
+    }
+
+private:
+    unordered_set<int> floydWarshall(int n, const vector<vector<int>>& graph) {
+        unordered_set<int> reachable;
+        for (const auto& connected : graph) {
+            reachable.emplace(connected[0] * n + connected[1]);
+        }
+        for (int k = 0; k < n; ++k) {
+            for (int i = 0; i < n; ++i) {
+                for (int j = 0; j < n; ++j) {
+                    if (!reachable.count(i * n + j) &&
+                        (reachable.count(i * n + k) && reachable.count(k * n + j))) {
+                        reachable.emplace(i * n + j);
+                    }
+                }
+            }
+        }
+        return reachable;
+    }
+};
+
+// Time:  O(n * q)
+// Space: O(p + n)
+class Solution_TLE {
+public:
+    vector<bool> checkIfPrerequisite(int n, vector<vector<int>>& prerequisites, vector<vector<int>>& queries) {
+        unordered_map<int, vector<int>> graph;
+        for (const auto& connected : prerequisites) {
+            graph[connected[0]].emplace_back(connected[1]);
+        }
+        vector<bool> result;
+        for (const auto& q : queries) {
+            vector<int> stk = {q[0]};
+            unordered_set<int> lookup = {q[0]};
+            while (!stk.empty()) {
+                const auto node = stk.back(); stk.pop_back();
+                for (const auto& nei : graph[node]) {
+                    if (lookup.count(nei)) {
+                        continue;
+                    }
+                    stk.emplace_back(nei);
+                    lookup.emplace(nei);
+                }
+            }
+            result.emplace_back(lookup.count(q[1]));
+        }
+        return result;
+    }
+};