From 1f0d3301a5f24a8265328482c09dca5c59c7cf37 Mon Sep 17 00:00:00 2001
From: atishaye <atishaye2000@gmail.com>
Date: Wed, 20 Oct 2021 18:19:19 +0530
Subject: [PATCH 1/4] add check_cycle.py

---
 graphs/check_cycle.py | 57 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)
 create mode 100644 graphs/check_cycle.py

diff --git a/graphs/check_cycle.py b/graphs/check_cycle.py
new file mode 100644
index 000000000000..45b5641a6a3b
--- /dev/null
+++ b/graphs/check_cycle.py
@@ -0,0 +1,57 @@
+"""
+Program to check if a cycle is present in a given graph
+"""
+
+
+def chk_cycle(graph: dict) -> bool:
+    """
+    Returns True if graph is cyclic else False
+
+    >>> graph1 = {0:[], 1:[0, 3], 2:[0, 4], 3:[5], 4:[5], 5:[]}
+    >>> chk_cycle(graph1)
+    False
+    >>> graph2 = {0:[1, 2], 1:[2], 2:[0, 3], 3:[3]}
+    >>> chk_cycle(graph2)
+    True
+    """
+    # Keep track of visited nodes
+    visited = set()
+    # To detect a back edge, keep track of vertices currently in the recursion stack
+    rec_stk = set()
+    for node in graph:
+        if node not in visited:
+            if depth_first_search(graph, node, visited, rec_stk):
+                return True
+    return False
+
+
+def depth_first_search(graph: dict, vertex: int, visited: set, rec_stk: set) -> bool:
+    """
+    Recur for all neighbours.
+    If any neighbour is visited and in rec_stk then graph is cyclic.
+
+    >>> graph = {0:[], 1:[0, 3], 2:[0, 4], 3:[5], 4:[5], 5:[]}
+    >>> vertex, visited, rec_stk = 0, set(), set()
+    >>> depth_first_search(graph, vertex, visited, rec_stk)
+    False
+    """
+    # Mark current node as visited and add to recursion stack
+    visited.add(vertex)
+    rec_stk.add(vertex)
+
+    for node in graph[vertex]:
+        if node not in visited:
+            if depth_first_search(graph, node, visited, rec_stk):
+                return True
+        elif node in rec_stk:
+            return True
+
+    # The node needs to be removed from recursion stack before function ends
+    rec_stk.remove(vertex)
+    return False
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()

From 3ac7fb179c156760c85151b6ea773e5201094f37 Mon Sep 17 00:00:00 2001
From: Atishaye Jain <64211411+atishaye@users.noreply.github.com>
Date: Fri, 22 Oct 2021 14:39:37 +0530
Subject: [PATCH 2/4] Update graphs/check_cycle.py

Co-authored-by: John Law <johnlaw.po@gmail.com>
---
 graphs/check_cycle.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/graphs/check_cycle.py b/graphs/check_cycle.py
index 45b5641a6a3b..177e75f2d67d 100644
--- a/graphs/check_cycle.py
+++ b/graphs/check_cycle.py
@@ -3,7 +3,7 @@
 """
 
 
-def chk_cycle(graph: dict) -> bool:
+def check_cycle(graph: dict) -> bool:
     """
     Returns True if graph is cyclic else False
 

From 54e473dde9df35f1bfaa1ba6931bdadb3b3d9f97 Mon Sep 17 00:00:00 2001
From: atishaye <atishaye2000@gmail.com>
Date: Fri, 22 Oct 2021 16:05:20 +0530
Subject: [PATCH 3/4] Update check_cycle.py

---
 graphs/check_cycle.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/graphs/check_cycle.py b/graphs/check_cycle.py
index 177e75f2d67d..68141aa4e853 100644
--- a/graphs/check_cycle.py
+++ b/graphs/check_cycle.py
@@ -8,10 +8,10 @@ def check_cycle(graph: dict) -> bool:
     Returns True if graph is cyclic else False
 
     >>> graph1 = {0:[], 1:[0, 3], 2:[0, 4], 3:[5], 4:[5], 5:[]}
-    >>> chk_cycle(graph1)
+    >>> check_cycle(graph1)
     False
     >>> graph2 = {0:[1, 2], 1:[2], 2:[0, 3], 3:[3]}
-    >>> chk_cycle(graph2)
+    >>> check_cycle(graph2)
     True
     """
     # Keep track of visited nodes

From e8dafec7610eac5e48669278631587f8ac4c9a5f Mon Sep 17 00:00:00 2001
From: Christian Clauss <cclauss@me.com>
Date: Sat, 23 Oct 2021 12:25:49 +0200
Subject: [PATCH 4/4] Apply suggestions from code review

---
 graphs/check_cycle.py | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/graphs/check_cycle.py b/graphs/check_cycle.py
index 68141aa4e853..71d42b4689b7 100644
--- a/graphs/check_cycle.py
+++ b/graphs/check_cycle.py
@@ -7,11 +7,9 @@ def check_cycle(graph: dict) -> bool:
     """
     Returns True if graph is cyclic else False
 
-    >>> graph1 = {0:[], 1:[0, 3], 2:[0, 4], 3:[5], 4:[5], 5:[]}
-    >>> check_cycle(graph1)
+    >>> check_cycle(graph={0:[], 1:[0, 3], 2:[0, 4], 3:[5], 4:[5], 5:[]})
     False
-    >>> graph2 = {0:[1, 2], 1:[2], 2:[0, 3], 3:[3]}
-    >>> check_cycle(graph2)
+    >>> check_cycle(graph={0:[1, 2], 1:[2], 2:[0, 3], 3:[3]})
     True
     """
     # Keep track of visited nodes