add directed_cycle.py

README.md

@@ -62,6 +62,7 @@ Try to keep the interface and variable name consistent with the original book wh
   * [DegreesOfSeparation](algs4/degrees_of_separation.py)
   * [Digraph](algs4/digraph.py)
   * [DirectedDFS](algs4/directed_dfs.py)
+  * [DirectedCycle](algs4/directed_cycle.py)
 
 * 5 STRING
 

algs4/directed_cycle.py

@@ -0,0 +1,72 @@
+"""
+  Execution:    python directed_cycle.py input.txt
+  Data files:   https://algs4.cs.princeton.edu/42digraph/tinyDG.txt
+                https://algs4.cs.princeton.edu/42digraph/tinyDAG.txt
+
+  Finds a directed cycle in a digraph.
+  Runs in O(E + V) time.
+
+  % python directed_cycle.py tinyDG.txt
+  Directed cycle: 3 5 4 3
+
+  %  python directed_cycle.py tinyDAG.txt
+  No directed cycle
+
+ """
+from algs4.stack import Stack
+from algs4.digraph import Digraph
+
+
+class DirectedCycle:
+
+    def __init__(self, G):
+        self._marked = [False for _ in range(G.V)]
+        self.edge_to = [False for _ in range(G.V)]
+        self.on_stack = [False for _ in range(G.V)]
+        self.cycle = None
+        for v in range(G.V):
+            if not self._marked[v]:
+                self.dfs(G, v)
+
+    def dfs(self, G, v):
+        self._marked[v] = True
+        self.on_stack[v] = True
+
+        for w in G.adj[v]:
+            if self.has_cycle():
+                return
+            if not self._marked[w]:
+                self.edge_to[w] = v
+                self.dfs(G, w)
+            elif self.on_stack[w]:
+                self.cycle = Stack()
+                x = v
+                while x != w:
+                    self.cycle.push(x)
+                    x = self.edge_to[x]
+                self.cycle.push(w)
+                self.cycle.push(v)
+        self.on_stack[v] = False
+
+    def marked(self, v):
+        return self._marked[v]
+
+    def has_cycle(self):
+        return self.cycle is not None
+
+if __name__ == '__main__':
+    import sys
+    f = open(sys.argv[1])
+    V = int(f.readline())
+    E = int(f.readline())
+    g = Digraph(V)
+    for i in range(E):
+        v, w = f.readline().split()
+        g.add_edge(v, w)
+
+    finder = DirectedCycle(g)
+    if finder.has_cycle():
+        for v in finder.cycle:
+            print(v, end=" ")
+    else:
+        print("No directed cycle")