CTCI Chapter 2 Complete

Python Solutions

Author: kyle8998

CTCI/Chapter2/2.6-Palindrome.py

@@ -1,5 +1,5 @@
-# CTCI 2.5
-# Sum Lists
+# CTCI 2.6
+# Palindrome
 
 import unittest
 #from LinkedList import LinkedList

CTCI/Chapter2/2.7-Intersection.py

@@ -1,5 +1,5 @@
-# CTCI 2.5
-# Sum Lists
+# CTCI 2.7
+# Intersection
 
 import unittest
 #from LinkedList import LinkedList

CTCI/Chapter2/2.8-Loop_Detection.py

@@ -0,0 +1,58 @@
+# CTCI 2.8
+# Loop Detection
+
+import unittest
+#from LinkedList import LinkedList
+
+# My Solution
+def detect_cycle(head):
+    nodes = []
+    curr = head
+    while curr:
+        if curr in nodes:
+            return curr
+        nodes.append(curr)
+        curr = curr.next
+        
+    return None
+
+#-------------------------------------------------------------------------------
+# CTCI Solution 
+def loop_detection(ll):
+    fast = slow = ll.head
+
+    while fast and fast.next:
+        fast = fast.next.next
+        slow = slow.next
+        if fast is slow:
+            break
+
+    if fast is None or fast.next is None:
+        return None
+
+    slow = ll.head
+    while fast is not slow:
+        fast = fast.next
+        slow = slow.next
+
+    return fast
+
+#-------------------------------------------------------------------------------
+#Testing
+
+class Node():
+  def __init__(self, data, next=None):
+    self.data, self.next = data, next
+
+class Test(unittest.TestCase):
+  def test_detect_cycle(self):
+    head1 = Node(100,Node(200,Node(300)))
+    self.assertEqual(detect_cycle(head1), None)
+    node1 = Node(600)
+    node2 = Node(700,Node(800,Node(900,node1)))
+    node1.next = node2
+    head2 = Node(500,node1)
+    self.assertEqual(detect_cycle(head2), node1)
+
+if __name__ == "__main__":
+  unittest.main()

README.md

@@ -116,7 +116,8 @@ Cracking the Coding Interview 6th Edition Python Solutions
 | 2.4|            | Partition                                                                                    | [Python](CTCI/Chapter2/2.4-Partition.py)                        |
 | 2.5|            | Sum Lists                                                                                    | [Python](CTCI/Chapter2/2.5-Sum_Lists.py)                        |
 | 2.6|            | Palindrome                                                                                   | [Python](CTCI/Chapter2/2.6-Palindrome.py)                       |
-| 2.7|            | Intersection                                                                                 | [Python](CTCI/Chapter2/2.7-Intersection.py)                       |
+| 2.7|            | Intersection                                                                                 | [Python](CTCI/Chapter2/2.7-Intersection.py)                     |
+| 2.8|            | Loop Detection                                                                               | [Python](CTCI/Chapter2/2.8-Loop_Detection.py)                   |
 
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