__Insertion - in a linked list__to insert a node at any position like at the end, first, or in between the nodes, we need a reference to the node of the list.

so first we need to find the reference to the node of the list. and if you don't know how to find the reference to the node of the list then first read this post.

After completing this tutorial you are able to

- Insert the node beginning of the list
- Insert the node in an empty list
- Insert the node at the end of the list
- Insert the node in between the nodes
- Insert the node after a node
- Insert the node before a node
- Insert the node at a given position

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__Insert the node beginning of the list__

to insert a node at the beginning of the list first we need to allocate a new node called temp as you see in the image given below.

after that, we point the linked part of the new node to the first node of the list. and then we point the start variable to the temp node. so it will become the first node of the list.

__Note:__the process and steps we follow are very important because if we reverse the order of steps to insert the node then we stuck in the infinite loop of the list.

def insert_in_beginning(self, data): temp = Node(data) temp.link = self.start self.start = temp

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__Insert the node in an empty list__

in an empty we don't have any node. so till now, our list is empty the reference to the first node is null or None.

first, we allocate a new node called temp. and then we point the reference of the node to the start variable.

def insert_at_end(self, data): temp = Node(data) if self.start is None: self.start = temp

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__Insert the node at the end of the list__

to insert a node at the end of the list first we need a reference to the last node of the list. after finding the reference to the last node of the list. first, we allocate a new node called 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

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__Insert the node in between the nodes.__

to insert a node in the between the nodes we need a reference to the node after we are going to insert the node. as you see in the image. and we allocate a new node called temp.

and then we store the reference of the next node of p into the temp node as you see in the image given below.

and then we store the reference of the temp node into the p node.

__Note:__the order of the steps to insert the new node between the nodes of the list is very important. if we reverse our steps then we stuck into an infinite loop.

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__Insert a node after a node__

like if we want to insert the node after a node that having value x as you see in the image given below.

for example, we need to insert the node after the node that having value x=56 then first we need to find the reference to the node that has the particular value.

and then we allocate a new node called temp.

then we store the reference of the newly allocated node into the p node.

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

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__Insert a node before a node__

as if we want to insert the node before a node that having value x. for example, we need to insert the node before the node that having value x=45. then first we need to find the reference to the predecessor of the node that having value x.

def insert_before(self, data, x): # If list is empty if self.start is None: print("List is empty") return # x is in first node , ew node is to be inserted before first node if x == self.start.info: temp = Node(data) temp.link = self.start self.start = temp return # Find reference to predecessor of node contanining x 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

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__Insert a node at a given position.__

like if we want to insert a node at the kth position in the list. then first we need to find the reference to the node at the k-1 position.

for example, if we want to insert the node at the position k=4 then we find the reference to the node at the 3rd position. as you see in the image given below.

then we store the reference of the newly allocated node to the k-1 node. as you see in the image given below.

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

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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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