Python Program For Merge Sort Of Linked Lists

# Python3 program to merge sort of linked list

# create Node using class Node.
class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

class LinkedList:
    def __init__(self):
        self.head = None
    
    # push new value to linked list
    # using append method
    def append(self, new_value):
        
        # Allocate new node
        new_node = Node(new_value)
        
        # if head is None, initialize it to new node
        if self.head is None:
            self.head = new_node
            return
        curr_node = self.head
        while curr_node.next is not None:
            curr_node = curr_node.next
            
        # Append the new node at the end
        # of the linked list
        curr_node.next = new_node
        
    def sortedMerge(self, a, b):
        result = None
        
        # Base cases
        if a == None:
            return b
        if b == None:
            return a
            
        # pick either a or b and recur..
        if a.data <= b.data:
            result = a
            result.next = self.sortedMerge(a.next, b)
        else:
            result = b
            result.next = self.sortedMerge(a, b.next)
        return result
    
    def mergeSort(self, h):
        
        # Base case if head is None
        if h == None or h.next == None:
            return h

        # get the middle of the list 
        middle = self.getMiddle(h)
        nexttomiddle = middle.next

        # set the next of middle node to None
        middle.next = None

        # Apply mergeSort on left list 
        left = self.mergeSort(h)
        
        # Apply mergeSort on right list
        right = self.mergeSort(nexttomiddle)

        # Merge the left and right lists 
        sortedlist = self.sortedMerge(left, right)
        return sortedlist
    
    # Utility function to get the middle 
    # of the linked list 
    def getMiddle(self, head):
        if (head == None):
            return head

        slow = head
        fast = head

        while (fast.next != None and 
               fast.next.next != None):
            slow = slow.next
            fast = fast.next.next
            
        return slow
        
# Utility function to print the linked list 
def printList(head):
    if head is None:
        print(' ')
        return
    curr_node = head
    while curr_node:
        print(curr_node.data, end = " ")
        curr_node = curr_node.next
    print(' ')
    
# Driver Code
if __name__ == '__main__':
    li = LinkedList()
    
    # Let us create a unsorted linked list
    # to test the functions created. 
    # The list shall be a: 2->3->20->5->10->15 
    li.append(15)
    li.append(10)
    li.append(5)
    li.append(20)
    li.append(3)
    li.append(2)
    
    # Apply merge Sort 
    li.head = li.mergeSort(li.head)
    print ("Sorted Linked List is:")
    printList(li.head)

# This code is contributed by Vikas Chitturi

# Python program for the above approach


# Node Class
class Node:
    
    def __init__(self,key):
        self.data=key
        self.next=None

# Function to merge sort
def mergeSort(head):
    if (head.next == None):
        return head
    
    mid = findMid(head)
    head2 = mid.next
    mid.next = None
    newHead1 = mergeSort(head)
    newHead2 = mergeSort(head2)
    finalHead = merge(newHead1, newHead2)
    return finalHead

# Function to merge two linked lists
def merge(head1,head2):
    merged = Node(-1)
    
    temp = merged
    # While head1 is not null and head2
    # is not null
    while (head1 != None and head2 != None):
        if (head1.data < head2.data):
            temp.next = head1
            head1 = head1.next
        else:
            temp.next = head2
            head2 = head2.next
        temp = temp.next
    
    # While head1 is not null
    while (head1 != None):
        temp.next = head1
        head1 = head1.next
        temp = temp.next
    
    # While head2 is not null
    while (head2 != None):
        temp.next = head2
        head2 = head2.next
        temp = temp.next
    
    return merged.next

# Find mid using The Tortoise and The Hare approach
def findMid(head):
    slow = head
    fast = head.next
    while (fast != None and fast.next != None):
        slow = slow.next
        fast = fast.next.next
    return slow

# Function to print list
def printList(head):
    while (head != None):
        print(head.data,end=" ")
        head=head.next

# Driver Code
head = Node(7)
temp = head
temp.next = Node(10);
temp = temp.next;
temp.next = Node(5);
temp = temp.next;
temp.next = Node(20);
temp = temp.next;
temp.next = Node(3);
temp = temp.next;
temp.next = Node(2);
temp = temp.next;

# Apply merge Sort
head = mergeSort(head);
print("
Sorted Linked List is: 
");

printList(head);
 
# This code is contributed by avanitrachhadiya2155