74 lines
2.1 KiB
Python
74 lines
2.1 KiB
Python
# 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())
|