Linked lists

Linked List Interview Prep

Spencer Moran11/20/13

Why Linked Lists?

• Simplicity• Little variation• Pointers and references

Singly Linked List

Almost all LL interview questions are based on singly linked lists

Doubly Linked List

Implementation

• Can be implemented as an ADT

• Often use templates• List is referred to by the

head, the first node in the list

• If you modify the list, you must return the head of the new list

Traversal

Insertion

Deletion

Linked List Question Strategies

• Use two (or more??) pointers to traverse the list

• Traverse pointers at different speeds• Use an additional data structure

Common Linked List Questions

Find the kth to last element in a singly linked list.

Strategy

• Use multiple pointers to traverse the list.• Return an error if the list is not at least k

nodes long.• Traverse the list with 2 pointers:– Pointer1 starts at head– Pointer2 starts k nodes in front of the head.– When pointer2 reaches the end of the list, return

pointer1

Common Linked List Questions

Determine if a singly linked list has a loop or cycle.

Strategy

• Use multiple pointers and traverse them at different speeds.

• Start both pointers at head:– Traverse the list, moving fast pointer twice as fast

as the slow pointer.– If fast pointer reaches the end of the list, there is

no cycle.– If the pointers ever are equal to one another,

there is a cycle.

BONUS QUESTION

Determine if a linked list has a cycle. If it does, return the element where the cycle begins.

Bonus Question #2

Find the length of the cycle without traversing it multiple times.

Common Linked List Questions

Reverse a singly linked list.

Strategy

• Use an additional data structure.• Traverse the list, pushing each node into the

stack.• Set the new head equal to the first element in

the list.• Starting at the new head, add the top element

from the stack until the stack is empty.

Other common questions

• Remove duplicates from a singly linked list• Merge sort• Remove element from middle of list (without

head)• Check if the linked list is a palindrome• Fold a singly linked list using no containers

Meeting Summary

We also covered a bunch of questions at the end of the meeting. These questions were mostly about linked lists, but we also started covering trees and decided that trees and graphs will be our next topic. The date of our next meeting will be arranged through the facebook group.