Reversing the Linked list Data strctures and algorithms

Reversing a linked list means to change the linked part of all nodes of a linked list to the pointing to the nodes that come before every node.

As you see we have a linked list that has four nodes and to reversing this list we need to point every node is linked part to the node that comes right before every node. and in the end, we update the start variable's position to the last node of the list. as you see in the image given below.

Method to reverse a linked list using links of nodes

to reverse a linked list using the links of nodes we need three references prev, p and next. 

in the beginning, we set the prev variable to None and set the reference of the first node of the linked list to variable p.

and then we start a loop till the value p does become None. means till we don't reach the last node of the list.

in every iteration, we set the reference of the node that comes after node p into the variable next.

and then we set the linked part of node p equal to the prev variable. 

after that, we store the reference of node p into variable prev. so now the variable prev points to the node that point by variable p.

and then we change the reference of node p using the variable next. that means now the variable p points to the node that point by variable next.

now, these conditions will run until we will not reach at the end of the list.

Now we reached at the end of the list because the linked part of node p is None. so now we update the self variable is value and point to the last node of the list.

so now the linked list is reversed using the links of every node. 

here is the python code to reverse a linked list using the links of every node.

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

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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