operations on the linked list to perform operations like insertion, deletion, sorting, searching on the linked list we need to find the reference to a particular node in a linked list.

so today we are going to learn how we can find the reference to the nodes to perform operations on a linked list.

After completing this tutorial you are able to learn
  • Reference to the last node
  • Reference to second last node
  • Reference to a node with particular info
  • Reference to the predecessor of a node with particular info
  • Reference to a node at a particular position

Finding a reference to the last node


Operations - Linked list in Data Structures and algorithms

as we know linked list contains the null or None value at the linked part of the last node of the list. so here is the code to find the reference to the last node of the linked list.

p = self.start
while p.link is not None:
    p = p.link

Note: here we first store the reference of the first node of the list in p variable and then we continue to the next using the while loop till the value of link part of a node not equal to None.


Finding a reference to the second last node

Operations - Linked list in Data Structures and algorithms


here is the python code to find the reference to the second last node of the list.

p = self.start
while p.link.link is not None:
    p = p.link

Note: here we run the while loop till the value of next to next link of the node equal to the null of None.

Finding a reference to a node with particular info


Operations - Linked list in Data Structures and algorithms


let's assume we need to find the node that contains the value x. for example, if we need to find the node that contains the value 30 then here is the python code to find the node with a particular value.

p = self.start
while p is not None:
    if p.info == x:
        break    p = p.link

Note: here we run the while loop till the last node of the list. and if we found the value of the node that equal to the value that we are searching then the break statement stops the while loop.

Finding a reference to the predecessor of a node with particular info


Predecessor: predecessor node is the node before the node that contains the particular value. as you see in the image given below.

Operations - Linked list in Data Structures and algorithms


Note: here we need to find the reference to the predecessor of the node that contains the value x=30.

here is the python code to find the reference to the predecessor of the node with particular info.

p = self.start
while p.link is not None:
    if p.link.info == x:
        break    p = p.link

here we run the loop till the last node of the linked list and at every node, we check that if the info part of the next to next node is equal to the value x or not. if the value is equal to the x then we break the loop.

Finding a reference to a node at a particular position


Operations - Linked list in Data Structures and algorithms


Here we need to find the node at position 3 as you see in the image given above.
here is the python code to find the node at a particular position.

p = self.start
i = 1
while i<k and p is not None:
    p = p.link
    i += 1

Note: here we run the while loop till the value of i is less than k and the last node to the list. and we increment the value of I at every iteration.

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()