3. Linked Lists

In this chapter, we continue to study implementations of the $\mathtt{List}$ interface, this time using pointer-based data structures rather than arrays. The structures in this chapter are made up of nodes that contain the list items. Using references (pointers), the nodes are linked together into a sequence. We first study singly-linked lists, which can implement $\mathtt{Stack}$ and (FIFO) $\mathtt{Queue}$ operations in constant time per operation and then move on to doubly-linked lists, which can implement $\mathtt{Deque}$ operations in constant time.

Linked lists have advantages and disadvantages when compared to array-based implementations of the $\mathtt{List}$ interface. The primary disadvantage is that we lose the ability to access any element using $\mathtt{get(i)}$ or $\mathtt{set(i,x)}$ in constant time. Instead, we have to walk through the list, one element at a time, until we reach the $\mathtt{i}$th element. The primary advantage is that they are more dynamic: Given a reference to any list node $\mathtt{u}$, we can delete $\mathtt{u}$ or insert a node adjacent to $\mathtt{u}$ in constant time. This is true no matter where $\mathtt{u}$ is in the list.