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

Author: kamyu104

Python/count-unreachable-pairs-of-nodes-in-an-undirected-graph.py

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+# Time:  O(n)
+# Space: O(n)
+
+# flood fill, bfs, math
+class Solution(object):
+    def countPairs(self, n, edges):
+        """
+        :type n: int
+        :type edges: List[List[int]]
+        :rtype: int
+        """
+        def bfs(adj, u, lookup):
+            q = [u]
+            lookup[u] = 1
+            result = 1
+            while q:
+                new_q = []
+                for u in q:
+                    for v in adj[u]:
+                        if lookup[v]:
+                            continue
+                        lookup[v] = 1
+                        result += 1
+                        new_q.append(v)
+                q = new_q
+            return result
+        
+        adj = [[] for _ in xrange(n)]
+        for u, v in edges:
+            adj[u].append(v)
+            adj[v].append(u)
+        lookup = [0]*n
+        result = 0
+        for u in xrange(n):
+            if lookup[u]:
+                continue
+            cnt = bfs(adj, u, lookup)
+            result += cnt*(n-cnt)
+            n -= cnt
+        return result