Add a node in ascending order

Question

def add(self, value):
    #write your code here

    if self.head == None:
        new_node = Node(value)
        self.head = new_node
        self.tail = self.head


    elif self.head.value > new_node.value:
        new_node = Node(value)
        new_node.value = value
        new_node.next = self.head
        self.head = new_node

    else:
        new_node = Node(value)
        self.tail.setNext(new_node)
        self.tail = new_node

    self.count += 1

Above is the code for my function add. I am trying to add Node objects into a linked list in ascending order so that when I print it it comes out as:

>>> x.add(8)
>>> x.add(7)
>>> x.add(3)
>>> x.add(-6)
>>> x.add(58)
>>> x.add(33)
>>> x.add(1)
>>> x.add(-88)
>>> print(x)
Head:Node(-88) Tail:Node(58)
List:-88 -6 1 3 7 8 33 58

But when I do it with my code above it prints it out as:

>>> print(x)
Head:Node(-88)
Tail:Node(1)
List:-88 -6 3 7 8 58 33 1

I'm 80% sure the problem is within the elif statement but I am not sure how to fix it to make go in ascending order.

Answer 1

The problem is in the last else. So let's see what is going on: number, action, result:

• 8 -> in the first if -> 8
• 7 -> in elif 8 > 7 -> 7,8
• 3,-6 -> same in elif -> -6, 3, 7, 8

• 58 -> last else -> -6, 3, 7, 8, 58

  and here comes the error when 33 comes and enters the last else again since it is bigger than head which is -6, and the result becomes -6, 3, 7, 8, 58, 33.

To fix this you need to loop through the list to see where 33 needs to be placed, you can't have a sorted list just by placing the items at the start or at the end.

def add(self, value):
    #write your code here

    #if list is empty
    if self.head == None:
        new_node = Node(value)
        self.head = new_node
        self.tail = self.head

    #elif value < head set new head
    elif self.head.value > value:
        new_node = Node(value)
        new_node.value = value
        new_node.next = self.head
        self.head = new_node


    #elif value > tail set new tail
    elif value > self.tail.value:
      new_node = Node(value)
      self.tail.setNext(new_node)
      self.tail = new_node
    # and finally you need to loop to find the sweet spot
    else:
      # we will start the search from the head
      current_node = self.head
      # while the value we wish to insert is bigger than the next one
      while value > current_node.next.value:
        # set the current one to the next one
        current_node = current_node.next
      # finally we reached a node which is smaller than the value we wish to insert
      # but its next node is bigger
      new_node = Node(value)
      # set the new nodes next to the bigger node
      new_node.next = current_node.next
      # and the curren't node's next to the new one
      current_node.next = new_node