Inserting Into Linked List Python

Satbir Kira :

I've create a pretty standard linked list in python with a Node class and LinkedList class. I've also added in methods for LinkedList as follows:

add(newNode): Adds an element to the linked list
addBefore(valueToFind, newNode): Adds a new node before an element with the value specified.
printClean: Prints the linked list

I'm trying to use the addBefore method to perform an insertion, however it will not work if the insertion isn't on the head. I'm not sure why.

class Node:
    def __init__(self, dataval =None):
        self.dataval = dataval
        self.nextval = None

class LinkedList:
    def __init__(self, headval =None):
        self.headval = headval

    def add(self, newNode):
        # The linked list is empty
        if(self.headval is None):
            self.headval = newNode
        else:
            # Add to the end of the linked list
            currentNode = self.headval
            while currentNode is not None:
                # Found the last element
                if(currentNode.nextval is None):
                    currentNode.nextval = newNode
                    break
                else:
                    currentNode = currentNode.nextval


    def addBefore(self, valueToFind, newNode):
        currentNode = self.headval
        previousNode = None
        while currentNode is not None:
            # We found the element we will insert before
            if (currentNode.dataval == valueToFind):
                # Set our new node's next value to the current element
                newNode.nextval = currentNode

                # If we are inserting at the head position
                if (previousNode is None):
                    self.headval = newNode
                else:
                    # Change previous node's next to our new node
                    previousNode.nexval = newNode
                    return 0

            # Update loop variables
            previousNode = currentNode
            currentNode = currentNode.nextval
        return -1

    def printClean(self):
        currentNode = self.headval
        while currentNode is not None:
            print(currentNode.dataval, end='')
            if(currentNode.nextval != None):
                print("->", end='')
                currentNode = currentNode.nextval
            else:
                return

testLinkedList = LinkedList()
testLinkedList.add(Node("Monday"))
testLinkedList.add(Node("Wednesday"))
testLinkedList.addBefore("Wednesday", Node("Tuesday"))
testLinkedList.printClean()

Monday->Wednesday

4l3j0 :

You have a typo, see the #TODO below in the method addBefore:

    def addBefore(self, valueToFind, newNode):
    currentNode = self.headval
    previousNode = None
    while currentNode is not None:
        # We found the element we will insert before
        if (currentNode.dataval == valueToFind):
            # Set our new node's next value to the current element
            newNode.nextval = currentNode

            # If we are inserting at the head position
            if (previousNode is None):
                self.headval = newNode
            else:
                # Change previous node's next to our new node
                previousNode.nexval = newNode #TODO: Fix Typo: nexval
                return 0

        # Update loop variables
        previousNode = currentNode
        currentNode = currentNode.nextval
    return -1