sorting a Linked list Data structures and algorithms

Sorting - linked list: there are many algorithms to sort a linked list like the bubble sort, merge sort, etc.

so today we are going to sort a linked list by exchanging the values of nodes using bubble sort algorithm.

Sorting a linked list using the bubble sort algorithm

In bubble to sort a linked list that has n items, we need to iterate over list n-1 times. like in the example we have a list of 5 items then we need to iterate over the list for 4 times.

to sort a linked list first we need three references p, q, and end. in the first iteration we set the end variable to None. and in every iteration, we set the p variable to the first node of the list and q variable to the second node of the list.



if in the iteration the value of the linked part of node p is equal to the end then we break the iteration and continue to next iteration.

if the info part of node p is greater than the info part of node q then we swap the values of node p and q otherwise we refer p and q to the next node of the list.
so in the given example list, the value of node p is not greater than the value of q node so we refer p and q to the next node of the list.



here also the value of node p is not greater than the value of node q so we go to the next nodes.



here the value of node p is greater than the value of node q so we swap them.



next, we go to the next nodes here the value of node p is not greater than the value of node q so we don't swap them.



now we go to the next nodes of the list. here the value of the linked part of node p is equal to the end variable. so here the first iteration ends.



now after the iteration one end, the last node of the list becomes sorted so we refer the end variable to the last node of the list.



now the second iteration starts and in second iteration we refer the variable p and q to the first and second node of the list and swap values of nodes p and q if the values of node p are greater than the value of node q.
this condition will run till the linked part of node p not become equal to the end variable. as you see in the images given below.











and after every iteration, we update the value of the variable end. because if we iterate the list over two times then the last two nodes of the list become sorted. so we don't need to check them again.

















so after performing the n-1 iterations over the linked list. where n is the size of the list. now our list becomes sorted.

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

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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