**as we know to for performing operations on the linked list we need to find the reference to the node of the list. so to delete a node from any position in the linked list we need to first find the reference of node.**

__Deletion - Linked list__and if you don't know how to find the reference of a node in the linked list then first complete read this post.

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__Deletion - Linked list in Data structures and algorithms__

- Deletion of the first node
- Deletion of the only node
- Deletion in between the node
- Deletion at the end of the list

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__Deletion of the first node from the linked list.__

as you see we have a linked list that contains 5 items in it.to delete the first node of the list we need to point the start variable of the linked list to the second node of the list.

so now the first node of the linked list is deleted.

def delete_first_node(self): if self.start is None: return self.start = self.start.link

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__Deletion of the only node from the linked list.__

like if we have a list that has only one node.then to delete the only node of the list we store the null or None value to the start variable.

so now our list becomes empty.

`self.start = None`

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__Deletion in between the nodes of a linked list__

to delete a node between the nodes of the linked list first we need to find the reference of the predecessor of the node that we want to delete.like in the example given below we want to delete between the second and fourth nodes. so we want to delete the node that contains the value 30.

so we need to find the reference to the node that comes before the node that contains the value 30.

then we refer the p node to the node that comes second after the node p. as you see in the image given below.

now the node is deleted from the list.

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__Deletion at the end of the linked list.__

to delete the last node we need a reference to the second last node of the linked list. as we see in the image given below.after that, we set the linked part of p to null or None.

so now the last node of the linked list deleted.

def delete_last_node(self): if self.start is None: return if self.start.link is None: self.start = None return p = self.start while p.link.link is not None: p = p.link p.link = None

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__Python program for traversing in the linked list.__

class Node: def __init__(self, value): self.info = value self.link = None class SingleLinkedList: def __init__(self): self.start = None def display_list(self): if self.start is None: print("List is empty") return else: print("List is : ") p = self.start while p is not None: print(p.info, " ", end=' ') p = p.link print() def count_nodes(self): p = self.start n = 0 while p is not None: n += 1 p = p.link print("Number of nodes in the list = ", n) def search(self): position = 1 p = self.start while p is not None: if p.info == x: print(x, " is at position ", position) return True position += 1 p = p.link else: print(x, " not found in list") return False def insert_in_beginning(self, data): temp = Node(data) temp.link = self.start self.start = temp def insert_at_end(self, data): temp = Node(data) if self.start is None: self.start = temp return p = self.start while p.link is not None: p = p.link p.link = temp def create_list(self): n = int(input("Enter the number of nodes : ")) if n == 0: return for i in range(n): data = int(input("Enter the element to be inserted : ")) self.insert_at_end(data) def insert_after(self, data, x): p = self.start while p is not None: if p.info == x: break p = p.link if p is None: print(x, "not present in the list") else: temp = Node(data) temp.link = p.link p.link = temp def insert_before(self, data, x): # If list is empty if self.start is None: print("List is empty") return if x == self.start.info: temp = Node(data) temp.link = self.start self.start = temp return p = self.start while p.link is not None: if p.link.info == x: break p = p.link if p.link is None: print(x, " not present in the list") else: temp = Node(data) temp.link = p.link p.link = temp def insert_at_position(self, data, k): if k == 1: temp = Node(data) temp.link = self.start self.start = temp return p = self.start i = 1 while i < k - 1 and p is not None: p = p.link i += 1 if p is None: print("You can insert only upto position", i) else: temp = Node(data) temp.link = p.link p.link = temp def delete_node(self, x): if self.start is None: print("List is empty") if self.start.info == x: self.start = self.start.link return p = self.start while p.link is not None: if p.link.info == x: break p = p.link if p.link is None: print("Element ", x , "not in list") else: p.link = p.link.link def delete_first_node(self): if self.start is None: return self.start = self.start.link def delete_last_node(self): if self.start is None: return if self.start.link is None: self.start = None return p = self.start while p.link.link is not None: p = p.link p.link = None def reverse_list(self): prev = None p = self.start while p is not None: next = p.link p.link = prev prev = p p = next self.start = prev def bubble_sort_exdata(self): end = None while end != self.start.link: p = self.start while p.link != end: q = p.link if p.info > q.info: p.info, q.info = q.info, p.info p = p.link end = p def bubble_sort_exlinks(self): end = None while end != self.start.link: r = p = self.start while p.link != end: q = p.link if p.info > q.info : p.link = q.link q.link = p if p != self.start: r.link = q else: self.start = q p,q = q,p r = p p = p.link end = p def has_cycle(self): if self.find_cycle() is None: return False else: return True def find_cycle(self): if self.start is None or self.start.link is None: return None slowR = self.start fastR = self.start while fastR is not None and fastR.link is not None: slowR = slowR.link fastR = fastR.link.link if slowR == fastR: return slowR return None def remove_cycle(self): c = self.find_cycle() if c is None: return print("Node at which the cycle was detected is ", c.info) p = c q = c len_cycle = 0 while True: len_cycle+=1 q = q.link if p == q: break print("Length of cycle is :", len_cycle) len_rem_list = 0 p = self.start while p != q: len_rem_list+=1 p = p.link q = q.link print("Number of nodes not included in the cycle are : ", len_rem_list) length_list = len_cycle + len_rem_list print("Length of the list is : ", length_list) p = self.start for i in range(length_list-1): p = p.link p.link = None def insert_cycle(self, x): if self.start is None: return p = self.start px = None prev = None while p is not None: if p.info == x: px = p prev = p p = p.link if px is not None: prev.link = px else: print(x, " not present in list") def merge2(self, list2): merge_list = SingleLinkedList() merge_list.start = self._merge2(self.start, list2.start) return merge_list def _merge2(self, p1, p2): if p1.info <= p2.info: startM = p1 p1 = p1.link else: startM = p2 p2 = p2.link pM = startM while p1 is not None and p2 is not None: if p1.info <= p2.info: pM.link = p1 pM = pM.link p1 = p1.link else: pM.link = p2 pM = pM.link p2 = p2.link if p1 is None: pM.link = p2 else: pM.link = p1 return startM def merge_sort(self): self.start = self._merge_sort_rec(self.start) def _merge_sort_rec(self, list_start): if list_start is None or list_start.link is None: return list_start start1 = list_start start2 = self._divide_list(list_start) start1 = self._merge_sort_rec(start1) start2 = self._merge_sort_rec(start2) startM = self._merge2(start1, start2) return startM def _divide_list(self, p): q = p.link.link while q is not None and q.link is not None: p = p.link q = q.link.link start2 = p.link p.link = None return start2 ########################## list = SingleLinkedList() list.create_list() while True: print("1. Display list") print("2. Count the number of nodes") print("3. Search for an element") print("4. Insert in empty list/insert in beginning of the list") print("5. Insert a node at the end of the list") print("6. Insert a node after a specified node") print("7. Insert a node before a specified node") print("8. Insert a node at a given position") print("9. Delete first node") print("10. Delete last node") print("11. Delete any node") print("12. Reverse the list") print("13. Bubble sort by exchanging data") print("14. Bubble sort by exchanging links") print("15. Merge sort") print("16. Insert Cycle") print("17. Detect Cycle") print("18. Remove Cycle") print("19. Quite") option = int(input("Enter your choice")) if option == 1: list.display_list() elif option == 2: list.count_nodes() elif option == 3: data = int(input("Enter the element to be searched : ")) list.search(data) elif option == 4: data = int(input("Enter the element to be inserted : ")) list.insert_in_beginning(data) elif option == 5: data = int(input("Enter the element to be inserted : ")) list.insert_at_end(data) elif option == 6: data = int(input("Enter the element to be inserted : ")) x = int(input("Enter the element after which to insert : ")) list.insert_after(data, x) elif option == 7: data = int(input("Enter the element to be inserted : ")) x = int(input("Enter the element before which to insert : ")) list.insert_before(data, x) elif option == 8: data = int(input("Enter the element to be inserted : ")) k = int(input("Enter the position at which to insert : ")) list.insert_at_position(data, k) elif option == 9: list.delete_first_node() elif option == 10: list.delete_last_node() elif option == 11: data = int(input("Enter the element to be deleted : ")) list.delete_node(data) elif option == 12: list.reverse_list() elif option == 13: list.bubble_sort_exdata() elif option == 14: list.bubble_sort_exlinks() elif option == 15: list.merge_sort() elif option == 16: data = int(input("Enter the element at which the cycle has to be inserted : ")) list.insert_cycle(data) elif option == 17: if list.has_cycle(): print("List has a cycle") else: print("List does not have a cycle") elif option == 18: list.remove_cycle() elif option == 19: break else: print("Wrong option") print()

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