Python Program For Flattening A Multilevel Linked List

Given a linked list where in addition to the next pointer, each node has a child pointer, which may or may not point to a separate list. These child lists may have one or more children of their own, and so on, to produce a multilevel data structure, as shown in the below figure. You are given the head of the first level of the list. Flatten the list so that all the nodes appear in a single-level linked list. You need to flatten the list in a way that all nodes at the first level should come first, then nodes of the second level, and so on.
Each node is a C struct with the following definition.

Python3

# A linked list node has data,
# next pointer and child pointer
class Node:
    def __init__(self, data):
        self.data = data
        self.next = None
        self.child = None

        # This code contributed by umadevi9616

The above list should be converted to 10->5->12->7->11->4->20->13->17->6->2->16->9->8->3->19->15

The problem clearly says that we need to flatten level by level. The idea of a solution is, we start from the first level, process all nodes one by one, if a node has a child, then we append the child at the end of the list, otherwise, we don’t do anything. After the first level is processed, all next-level nodes will be appended after the first level. The same process is followed for the appended nodes.

1) Take the "cur" pointer, which will point to the head 
        of the first level of the list
2) Take the "tail" pointer, which will point to the end of 
   the first level of the list
3) Repeat the below procedure while "curr" is not NULL.
    I) If the current node has a child then
    a) Append this new child list to the "tail"
        tail->next = cur->child
    b) Find the last node of the new child list and update 
       the "tail"
        tmp = cur->child;
        while (tmp->next != NULL)
            tmp = tmp->next;
        tail = tmp;
    II) Move to the next node. i.e. cur = cur->next

Following is the implementation of the above algorithm. 

Python3

# Python3 Program to flatten list with
# next and child pointers

# A linked list node has data,
# next pointer and child pointer
class Node:
    def __init__(self, data):
        self.data = data
        self.next = None
        self.child = None

# Return Node
def newNode(data):
    return Node(data)

# The main function that flattens
# a multilevel linked list
def flattenlist(head):

    # Base case
    if not head:
        return

    # Find tail node of first level
    # linked list
    temp = head
    while(temp.next != None):
        temp = temp.next
    currNode = head

    # One by one traverse through all
    # nodes of first level linked list
    # till we reach the tail node
    while(currNode != temp):

        # If current node has a child
        if(currNode.child):

            # then append the child
            # at the end of current list
            temp.next = currNode.child

            # and update the tail to new
            # last node
            tmp = currNode.child
            while(tmp.next):
                tmp = tmp.next
            temp = tmp

        # Change current node
        currNode = currNode.next

# A utility function to print
# all nodes of a linked list
def printList(head):
    if not head:
        return
    while(head):
        print("{}".format(head.data),
              end = " ")
        head = head.next

# Driver code
if __name__=='__main__':

    # Child list of 13
    child13 = newNode(16)
    child13.child = newNode(3)

    # Child List of 10
    head1 = newNode(4)
    head1.next = newNode(20)

    # Child of 20
    head1.next.child = newNode(2)
    head1.next.next = newNode(13)
    head1.next.next.child = child13

    # Child of 9
    child9 = newNode(19)
    child9.next = newNode(15)

    # Child List of 17
    child17 = newNode(9)
    child17.next = newNode(8)
    child17.child = child9

    # Child List of 7
    head2 = newNode(17)
    head2.next = newNode(6)
    head2.child = child17

    # Main List
    head = newNode(10)
    head.child = head1
    head.next = newNode(5)
    head.next.next = newNode(12)
    head.next.next.next = newNode(7)
    head.next.next.next.child = head2
    head.next.next.next.next = newNode(11)

    flattenlist(head)

    print("Flattened list is: ", end = "")
    printList(head)
# This code is contributed by 0_hero

Output:

10 5 12 7 11 4 20 13 17 6 2 16 9 8 3 19 15

Time Complexity: Since every node is visited at most twice, the time complexity is O(n) where n is the number of nodes in given linked list.

Space Complexity : O(1) 

Please refer complete article on Flatten a multilevel linked list for more details!