solves Longest Cycle in Graph (#2360) in python

Author: sumanth-botlagunta

python/longest_cycle_in_graph.py

@@ -0,0 +1,28 @@
+# https://leetcode.com/problems/longest-cycle-in-a-graph/description/
+# T:O(N) where N is the number of nodes in the graph
+# S:O(N) where N is the number of nodes in the graph
+
+class Solution:
+    def __init__(self):
+        self.answer = -1
+    
+    def dfs(self, node, edges, dist, visit):
+        visit[node] = True
+        neighbor = edges[node]
+        
+        if neighbor != -1 and not visit[neighbor]:
+            dist[neighbor] = dist[node] + 1
+            self.dfs(neighbor, edges, dist, visit)
+        elif neighbor != -1 and neighbor in dist:
+            self.answer = max(self.answer, dist[node] - dist[neighbor] + 1)
+    
+    def longestCycle(self, edges):
+        n = len(edges)
+        visit = [False] * n
+        
+        for i in range(n):
+            if not visit[i]:
+                dist = {i: 1}
+                self.dfs(i, edges, dist, visit)
+        
+        return self.answer

python/reorder_routes_to_make_all_paths_lead_to_city_zero.py

@@ -0,0 +1,26 @@
+# https://leetcode.com/problems/reorder-routes-to-make-all-paths-lead-to-the-city-zero/description/
+# T: O(N) where N is the number of nodes in the graph
+# S: O(N) where N is the number of nodes in the graph
+
+class Solution:
+    def __init__(self):
+        self.reorders = 0
+
+    def minReorder(self, n, connections):
+        edges = {(u, v) for u, v in connections}
+        graph = {i:[] for i in range(n)}
+        for u, v in connections:
+            graph[u].append(v)
+            graph[v].append(u)
+        visit = set()
+        visit.add(0)
+        def dfs(graph, edges, visit, city):
+            for node in graph[city]:
+                if node in visit:
+                    continue
+                if (node, city) not in edges:
+                    self.reorders+=1
+                visit.add(node)
+                dfs(graph, edges, visit, node)
+        dfs(graph, edges, visit, 0)
+        return self.reorders