Day 2 Double pointers in the Linked List

I was busy with dealing with something else the other day, but I will continue in this blog from now on :)

This time I will talk about another technique often used in a single-linked-list: double pointers. 

For example, when I need to check the nth last element in a linked list, I can use this technique to save memory (instead of creating an entire parallel list)

we will use one pointer to iterate through the entire list, but we’ll also move a second pointer delayed n steps behind the first one. A count variable can keep track of the position of the current element in the linked list that the tail pointer is pointing to, which is initially set to 1 which is the first element’s position.

 

nth last pointer = None
tail pointer = linked list head
count = 1

while tail pointer exists
  move tail pointer forward
  increment count

  if count >= n + 1
    if nth last pointer is None
      set nth last pointer to head
    else
      move nth last pointer forward

return nth last pointer

Above is the pseudecode for this approach.

The two pointers can move in different speeds.

For example, if we try to find the middle element of a linked list:

fast pointer = linked list head
slow pointer = linked list head
while fast pointer is not None
  move fast pointer forward
  if the end of the linked list has not been reached
    move fast pointer forward
    move slow pointer forward
return slow pointer