loop detection for linked lists

Author: nickolasteixeira

Python/chapter02/2.8 - Loop Detection/Nick.py

@@ -0,0 +1,114 @@
+#!/usr/bin/python3
+import unittest
+
+
+class Node:
+    def __init__(self, data, next=None):
+        self.data = data
+        self.next = next
+
+
+class SinglyLinkedList:
+    def __init__(self):
+        self.head = None
+
+    def addNode(self, data, next=None):
+        node = Node(data, next)
+        if not self.head:
+            self.head = node
+        else:
+            temp = self.head
+            while temp.next:
+                temp = temp.next
+            temp.next = node
+        print('Node added to LL-->', data)
+
+    def findNode(self, value):
+        if not self.head:
+            print('No list to iterate')
+            return
+
+        temp = self.head
+        while temp:
+            if temp.data == value:
+                return temp
+            temp = temp.next
+        return None
+
+    def printList(self):
+        temp = self.head
+        while temp:
+            print('->', temp.data)
+            temp = temp.next
+
+    def loopDetection(self):
+        if not self.head:
+            print('No list to iterate for loop detection')
+            return
+
+        slow = self.head
+        fast = self.head
+
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+            if slow == fast:
+                break
+
+        if not fast or not fast.next:
+            return None
+
+        slow = self.head
+        while slow is not fast:
+            slow = slow.next
+            fast = fast.next
+
+        return fast
+
+
+class Test(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        cls.l = SinglyLinkedList()
+        cls.l.addNode(3)
+        cls.l.addNode(1)
+        cls.l.addNode(5)
+        cls.l.addNode(9)
+        cls.l.addNode(7)
+        cls.l.addNode(2)
+        cls.l.addNode(10)
+        cls.l.printList()
+
+        cls.k = SinglyLinkedList()
+        cls.k.addNode(1)
+        cls.k.addNode(10)
+        cls.k.addNode(15)
+        cls.k.addNode(16)
+        cls.k.addNode(1)
+        cls.k.printList()
+
+    @classmethod
+    def tearDownClass(cls):
+        pass
+
+    def printStatement(self, node):
+        if node:
+            print("Start of Loop", hex(id(node)), '-->', node.data)
+        else:
+            print('No loop detected')
+
+    def test1(self):
+        node = self.l.findNode(10)
+        node.next = self.l.findNode(5)
+        # if uncomment below, will print in infinite
+        # self.l.printList()
+        start = self.l.loopDetection()
+        self.printStatement(start)
+
+    def test2(self):
+        start = self.k.loopDetection()
+        self.printStatement(start)
+
+
+if __name__ == '__main__':
+    unittest.main()