Create count-unreachable-pairs-of-nodes-in-an-undirected-graph.cpp

Author: kamyu104

C++/count-unreachable-pairs-of-nodes-in-an-undirected-graph.cpp

@@ -0,0 +1,46 @@
+// Time:  O(n)
+// Space: O(n)
+
+// flood fill, bfs, math
+class Solution {
+public:
+    long long countPairs(int n, vector<vector<int>>& edges) {
+        vector<vector<int>> adj(n);
+        for (const auto& e : edges) {
+            adj[e[0]].emplace_back(e[1]);
+            adj[e[1]].emplace_back(e[0]);
+        }
+        vector<bool> lookup(n);
+        const auto& bfs = [&](int u) {
+            vector<int> q = {u};
+            lookup[u] = true;
+            int64_t result = 1;
+            while (!empty(q)) {
+                vector<int> new_q;
+                for (const auto& u : q) {
+                    for (const auto& v : adj[u]) {
+                        if (lookup[v]) {
+                            continue;
+                        }
+                        lookup[v] = true;
+                        ++result;
+                        new_q.emplace_back(v);
+                    }
+                }
+                q = move(new_q);
+            }
+            return result;
+        };
+    
+        int64_t result = 0;
+        for (int u = 0; u < size(adj); ++u) {
+            if (lookup[u]) {
+                continue;
+            }
+            const int64_t cnt = bfs(u);
+            result += cnt * (n - cnt);
+            n -= cnt;
+        }
+        return result;
+    }
+};