From 2da3cda5b9e4dea18025bb6713352718fabb5ebe Mon Sep 17 00:00:00 2001
From: Seemant Tripathi <47049137+seemanttripathi@users.noreply.github.com>
Date: Fri, 21 Oct 2022 08:18:12 +0530
Subject: [PATCH] chore(Python): add cycle detection and removal in linkedlist
 (#1022)

---
 algorithms/Python/README.md                   |  1 +
 .../cycle-detection-and-removal-linkedlist.py | 73 +++++++++++++++++++
 2 files changed, 74 insertions(+)
 create mode 100644 algorithms/Python/linked_lists/cycle-detection-and-removal-linkedlist.py

diff --git a/algorithms/Python/README.md b/algorithms/Python/README.md
index a2949840..3bb3fa88 100644
--- a/algorithms/Python/README.md
+++ b/algorithms/Python/README.md
@@ -14,6 +14,7 @@
 - [Singly](linked_lists/singly.py)
 - [Reverse List](linked_lists/reverse-linkedlist.py)
 - [Middle Node](linked_lists/middle-node-linkedlist.py)
+- [Cycle Detection and Removal](linked_lists/cycle-detection-and-removal-linkedlist.py)
 
 
 ## Dictionaries
diff --git a/algorithms/Python/linked_lists/cycle-detection-and-removal-linkedlist.py b/algorithms/Python/linked_lists/cycle-detection-and-removal-linkedlist.py
new file mode 100644
index 00000000..94b263b7
--- /dev/null
+++ b/algorithms/Python/linked_lists/cycle-detection-and-removal-linkedlist.py
@@ -0,0 +1,73 @@
+# Topic - Linkedlist
+# Language - Python
+# Problem - Detection and Removal of Cycle in Linkedlist.
+# 
+# Idea - run 2 pointers at the start of the linkedlist
+# increment slow pointer by one and fast pointer by two nodes.
+# if slow pointer is equal to fast pointer - then cycle exist
+# to remove the cycle - we need to start slow pointer to start
+# and another fast pointer to same location it is pointing to
+# we will stop increamenting slow pointer when slow equals to
+# fast pointer and break the link from fast.next is None. 
+
+class Node:
+    def __init__(self, val):
+        self.val = val
+        self.next = None
+
+class Linkedlist:
+    def __init__(self):
+        self.head = None
+
+    # pushAtStart is used to create linkedlist
+    def pushAtStart(self, data):
+        node = Node(data)
+        node.next = self.head
+        self.head = node
+    
+
+    def cycleDetectionAndRemoval(self):
+        temp = self.head
+        if temp is None or temp.next is None:
+            return 'No Cycle Detected'
+        slow_ptr = temp
+        fast_ptr = temp
+
+        while (slow_ptr and fast_ptr and fast_ptr.next):
+            # moving slow pointer by one node and fast by two node.
+            slow_ptr = slow_ptr.next
+            fast_ptr = fast_ptr.next.next
+
+            # if slow equals to fast, then we have encountered cycle
+            if slow_ptr == fast_ptr:
+                slow_ptr = temp
+
+                while slow_ptr != fast_ptr:
+                    slow_ptr = slow_ptr.next
+                    fast_ptr = fast_ptr.next
+
+                # removing cycle
+                fast_ptr.next = None
+                return 'Cycle Detected and Removed'
+        return 'No Cycle Detected'
+
+if __name__ == '__main__':
+
+    # creating linkedlist
+    mylist = Linkedlist()
+
+    # adding nodes
+    mylist.pushAtStart(10)
+    mylist.pushAtStart(20)
+    mylist.pushAtStart(30)
+    mylist.pushAtStart(40)
+    mylist.pushAtStart(50)
+
+    # creating cycle
+    mylist.head.next.next.next.next.next = mylist.head.next.next
+
+    # removing cycle
+    print(mylist.cycleDetectionAndRemoval())
+
+    # after removing cycle we don't have cycle
+    print(mylist.cycleDetectionAndRemoval())